RC-5, NEC, JVC and Panasonic infrared codes

Wednesday, 3rd June 2009

I've rewritten the remote control signal decoding software to handle multiple protocols. As well as SIRCS, it now supports RC-5, NEC, JVC and two Panasonic codes (one "old" 11-bit code and one "new" 48-bit code). There's not much in the way of screenshots at the moment, other than a debug window that gets filled when keys are pressed:

NecCommand Address=27526, Command=5, Extended=True, Repeat=1
NecCommand Address=24, Command=87, Extended=False, Repeat=1
RC5Command Address=20, Command=53, Repeat=True, Repeat=1
NewPanasonicCommand OEM Device 1=2, OEM Device 2=32, Device=144, Sub-Device=0, Command=10, Repeat=1
RC5Command Address=8, Command=35, Repeat=True, Repeat=1
OldPanasonicCommand Address=0, Command=20, Repeat=1
SircsCommand Address=2362, Command=121, Length=20, Repeat=1
SircsCommand Address=7002, Command=84, Length=20, Repeat=1
NecCommand Address=64, Command=146, Extended=False, Repeat=1
NecCommand Address=81, Command=8, Extended=False, Repeat=1
JvcCommand Address=3, Command=23, Repeat=1

Gripping stuff, I'm sure you'll agree.

The C# source code for this can be downloaded here.

A keyring remote control (courtesy of Poundland) has highlighted one possible issue in handling repeating buttons. Rather than target any particular device, it will try and brute-force a response. For example, here's the result of pressing the power button once in one particular mode:

SircsCommand Address=1, Command=21, Length=12, Repeat=1
SircsCommand Address=1, Command=21, Length=12, Repeat=2
OldPanasonicCommand Address=0, Command=32, Repeat=1
NecCommand Address=32, Command=11, Extended=False, Repeat=1
NewPanasonicCommand OEM Device 1=2, OEM Device 2=32, Device=128, Sub-Device=0, Command=61, Repeat=1

That's four different protocols from one button. I suppose some sort of mapping from protocol-specific code to a string (so those five commands would be translated into five "power" strings) and comparing the time between signals to turn the input into something meaningful may help, but that would require an enormous database of known codes.

Remote controlling Windows the Sony way

Wednesday, 27th May 2009

It's been a while since I last posted, and unfortunately this post is to do with Sony remote controls again. rolleyes.gif

This time I'm attempting to use Sony (or compatible) remote controls to control software running on a Windows PC. I've recently been watching more films in PowerDVD, and some of the keyboard shortcuts (eg Ctrl+P for the menu) are a little difficult to hit in the dark and from a distance. I have a ready supply of universal remote controls as well as the PlayStation 2 DVD remote control, all of which work with the SIRCS protocol.

Serial port infra-red receiver built into an old TI GraphLink cable
Serial port infra-red receiver built into an old TI GraphLink cable

First up is the required hardware. This involves an infrared demodulator connected to a free serial port. I chose the serial port as .NET provides a way to handle pin change events and you do not need administrator rights to access it (as per the parallel port). I also had a broken Texas Instruments GraphLink cable that could be ripped apart to act as a case.

Infra-red receiver module schematic
Infra-red receiver module schematic

The circuit is pretty simple. Pin 4 (DTR) and 5 (GND) from the serial port form the power supply. DTR can be set to either +12V or -12V, so a rectifier diode is used to keep the input voltage above 0V. Following that is a reverse-biased zener diode and resistor to regulate the voltage below 5.1V. Finally, the output pin of the infra-red demodulator is connected to the input pin 8 (CTS) of the serial port.

Infra-red receiver module assembled on stripboard
Infra-red receiver module assembled on stripboard

The software handles the SerialPort.PinChanged event to time the length of input pulses. Once it detects a start bit (2.4mS) it starts decoding the rest of the command. When it's finished receiving a command it fires an event of its own, which the main software can react to.

using System;
using System.Diagnostics;
using System.IO.Ports;

namespace BeeDevelopment.Sircs {

    /// <summary>
    /// Represents a command sent by a SIRCS remote control.
    /// </summary>
    public struct SircsCommand : IEquatable<SircsCommand> {

        #region Properties

        private byte command;
        /// <summary>
        /// Gets or sets the command value.
        /// </summary>
        public byte Command {
            get { return this.command; }
            set { this.command = value; }
        }

        private short device;
        /// <summary>
        /// Gets or sets the device identifier.
        /// </summary>
        public short Device {
            get { return this.device; }
            set { this.device = value; }
        }

        private int length;
        /// <summary>
        /// Gets or sets the length of the command in bits.
        /// </summary>
        public int Length {
            get { return this.length; }
            set { this.length = value; }
        }

        #endregion

        #region Construction

        /// <summary>
        /// Creates an instance of a <see cref="SircsCommand"/> structure.
        /// </summary>
        /// <param name="command">The command value.</param>
        /// <param name="device">The device identifier.</param>
        /// <param name="length">The length of the command in bits.</param>
        public SircsCommand(byte command, short device, int length) {
            this.command = command;
            this.device = device;
            this.length = length;
        }

        #endregion

        #region Methods

        /// <summary>
        /// Converts the <see cref="SircsCommand"/> into a string.
        /// </summary>
        /// <returns>A string representation of the <see cref="SircsCommand"/>.</returns>
        public override string ToString() {
            return string.Format("Command={0:X2}, Device={1:X4}, Length={2}", this.command, this.device, this.length);
        }

        /// <summary>
        /// Returns the hash code for this instance.
        /// </summary>
        /// <returns>The hash code for this instance.</returns>
        public override int GetHashCode() {
            return this.command ^ this.device ^ this.length;
        }

        /// <summary>
        /// Returns a value indicating whether this instance is equal to another <see cref="SircsCommand"/> instance.
        /// </summary>
        /// <param name="other">The instance to compare to this one for equality.</param>
        /// <returns>True if the instances are equal, false otherwise.</returns>
        public bool Equals(SircsCommand other) {
            return this.command == other.command && this.device == other.device && this.length == other.length;
        }

        /// <summary>
        /// Returns a value indicating whether this instance is equal to another <see cref="SircsCommand"/> instance.
        /// </summary>
        /// <param name="other">The instance to compare to this one for equality.</param>
        /// <returns>True if the instances are equal, false otherwise.</returns>
        public override bool Equals(object other) {
            return other != null && other is SircsCommand && ((SircsCommand)other).Equals(this);
        }

        #endregion

        #region Operators

        public static bool operator ==(SircsCommand a, SircsCommand b) { return a.Equals(b); }
        public static bool operator !=(SircsCommand a, SircsCommand b) { return !a.Equals(b); }

        #endregion

    }

    #region Events

    /// <summary>
    /// Represents the method that will handle the <c>SircsCommandReceived</c> event.
    /// </summary>
    /// <param name="sender">The object that fired the event.</param>
    /// <param name="e">Information about the event.</param>
    public delegate void SircsCommandReceivedEventHandler(object sender, SircsCommandReceivedEventArgs e);

    /// <summary>
    /// Provides data for the <c>SircsReceived.SircsCommandReceived</c> event.
    /// </summary>
    public class SircsCommandReceivedEventArgs : EventArgs {

        #region Properties

        /// <summary>
        /// Gets the <see cref="SircsCommand"/> that was received.
        /// </summary>
        public SircsCommand Command { get; private set; }

        /// <summary>
        /// Gets the number of times that the incoming command has been repeated when held.
        /// </summary>
        public int Repeat { get; private set; }

        #endregion

        #region Construction

        /// <summary>
        /// Creates a <see cref="SircsCommandReceivedEventArgs"/> instance.
        /// </summary>
        /// <param name="command">The <see cref="SircsCommand"/> that was recieved.</param>
        /// <param name="repeat">The number of times that the incoming command has been repeated when held.</param>
        public SircsCommandReceivedEventArgs(SircsCommand command, int repeat) {
            this.Command = command;
            this.Repeat = repeat;
        }

        #endregion

        #region Methods

        /// <summary>
        /// Converts the <see cref="SircsCommandReceivedEventArgs"/> into a string.
        /// </summary>
        /// <returns>A string representation of the <see cref="SircsCommandReceivedEventArgs"/>.</returns>
        public override string ToString() {
            return string.Format("{0}, Repeat={1}", this.Command, this.Repeat);
        }

        #endregion
    }

    #endregion

    /// <summary>
    /// Provides a way to receive SIRCS commands from a simple receiver attached to a serial port.
    /// </summary>
    public class SircsReceiver : IDisposable {

        #region Constants

        /// <summary>
        /// The minimum time length for a start bit (nominally 2.4ms).
        /// </summary>
        private const double StartBitMinLength = 2.0E-3;

        /// <summary>
        /// Threshold time length between a "low" (0.6ms) and a "high" (1.2ms) bit.
        /// </summary>
        private const double DataBitLengthThreshold = 0.9E-3;

        /// <summary>
        /// The maximum time length between data bits. If this is exceeded, any data command transfer is cancelled.
        /// </summary>
        private const double IntraBitMaxLength = 0.8E-3;

        /// <summary>
        /// The maximum time length between repeating commands. Commands are supposed to repeat every 45ms.
        /// </summary>
        private const double RepeatCommandMaxLength = 120.0E-3;

        #endregion

        #region Private Fields

        /// <summary>
        /// The <see cref="SerialPort"/> that the receiver is connected to.
        /// </summary>
        private SerialPort Port = null;

        /// <summary>
        /// The last time that the pin state changed in ticks.
        /// </summary>
        private long LastPinChangedTime = 0;

        /// <summary>
        /// A <see cref="Stopwatch"/> instance used to time incoming bits.
        /// </summary>
        private Stopwatch BitTimer = null;

        /// <summary>
        /// Set to <c>true</c> when receiving a command, <c>false</c> otherwise.
        /// </summary>
        private bool ReceivingCommand = false;

        /// <summary>
        /// Counts the number of bits currently received.
        /// </summary>
        private int BitsReceived = 0;

        /// <summary>
        /// Stores the command as it gets built up.
        /// </summary>
        private uint Command = 0;

        /// <summary>
        /// Stores the last received command.
        /// </summary>
        private SircsCommand LastCommand = default(SircsCommand);

        /// <summary>
        /// Stores the number of times the received command has been repeated.
        /// </summary>
        private int LastCommandRepeatCount = 0;

        /// <summary>
        /// A <see cref="Stopwatch"/> instance used to time repeating commands.
        /// </summary>
        private Stopwatch RepeatTimer = null;

        #endregion

        #region Construction/Destruction

        /// <summary>
        /// Creates an instance of a <see cref="SircsReceiver"/> from a serial port name.
        /// </summary>
        /// <param name="portName">The name of the serial port the receiver is connected to.</param>
        public SircsReceiver(string portName) {

            // Set up the serial port.
            this.Port = new SerialPort(portName);
            this.Port.PinChanged += new SerialPinChangedEventHandler(PinChanged);

            // Open the port for access.
            this.Port.Open();
            this.Port.DtrEnable = true;
            this.Port.RtsEnable = true;

            // Get the timers running.
            this.BitTimer = new Stopwatch();
            this.BitTimer.Start();
            this.RepeatTimer = new Stopwatch();
            this.RepeatTimer.Start();
        }

        /// <summary>
        /// Releases the resources used by this <see cref="SircsReceiver"/> instance.
        /// </summary>
        public void Dispose() {
            if (this.Port != null) {
                this.Port.PinChanged -= new SerialPinChangedEventHandler(PinChanged);
                this.Port.Dispose();
                this.Port = null;
            }
        }

        ~SircsReceiver() {
            this.Dispose();
        }

        #endregion

        #region Events

        /// <summary>
        /// An event that is fired when a <see cref="SircsCommand"/> is received.
        /// </summary>
        public event SircsCommandReceivedEventHandler SircsCommandReceived;

        /// <summary>
        /// A method that is invoked when a <see cref="SircsCommand"/> is received.
        /// </summary>
        /// <param name="e"></param>
        protected virtual void OnSircsCommandReceived(SircsCommandReceivedEventArgs e) {
            if (this.SircsCommandReceived != null) this.SircsCommandReceived(this, e);
        }

        #endregion

        #region SIRCS protocol handling

        void PinChanged(object sender, SerialPinChangedEventArgs e) {

            // Respond to changes on the CTS pin.
            if (e.EventType == SerialPinChange.CtsChanged) {

                // Quickly grab the current time and current CTS level.
                long CurrentPinChangedTime = this.BitTimer.ElapsedTicks;
                bool CurrentLevel = this.Port.CtsHolding;

                // Calculate the time elapsed.
                long DeltaTime = CurrentPinChangedTime - this.LastPinChangedTime;
                double SecondsElapsed = (double)DeltaTime / (double)Stopwatch.Frequency;
                this.LastPinChangedTime = CurrentPinChangedTime;

                if (CurrentLevel) { // If the current signal level is high, we may assume that we've just timed a low pulse.

                    // Have we received a start bit?
                    if (SecondsElapsed > SircsReceiver.StartBitMinLength) {
                        this.ReceivingCommand = true;
                        this.BitsReceived = 0;
                        this.Command = 0;
                    } else if (this.ReceivingCommand) {
                        // Process incoming bit.
                        this.Command >>= 1;
                        if (SecondsElapsed > SircsReceiver.DataBitLengthThreshold) {
                            this.Command |= unchecked((uint)(1 << 31));
                        }

                        // Have we received enough bits?
                        switch (++this.BitsReceived) {
                            case 12:
                            case 15:
                            case 20:
                                // We've received enough bits to handle the input as a received command.
                                // Check to see if there's any more data forthcoming.
                                long EndTime = CurrentPinChangedTime + (long)(Stopwatch.Frequency * SircsReceiver.IntraBitMaxLength);
                                while (BitTimer.ElapsedTicks < EndTime) {
                                    if (!(CurrentLevel = this.Port.CtsHolding)) break;
                                }
                                // The input is still high - there's no more data coming in; we've received a command.
                                if (CurrentLevel) {
                                    // Construct a struct to hold information about the recieved data.
                                    SircsCommand ReceivedCommand = new SircsCommand(
                                        (byte)((this.Command >> (32 - this.BitsReceived)) & 0x7F),
                                        (short)(this.Command >> ((32 + 7) - this.BitsReceived)),
                                        this.BitsReceived
                                    );
                                    
                                    // Reset the timer.
                                    this.ReceivingCommand = false;
                                    this.BitTimer.Reset();
                                    this.BitTimer.Start();
                                    this.LastPinChangedTime = 0;
                                    
                                    // Calculate the repeat count.

                                    // Quickly grab the current time and current CTS level.
                                    long RepeatTimeTicks = this.RepeatTimer.ElapsedTicks;
                                    this.RepeatTimer.Reset();
                                    this.RepeatTimer.Start();

                                    // Calculate the repeat time elapsed.
                                    double RepeatTimeSeconds = (double)RepeatTimeTicks / (double)Stopwatch.Frequency;

                                    // Is the command repeating?
                                    if (ReceivedCommand == this.LastCommand && RepeatTimeSeconds < SircsReceiver.RepeatCommandMaxLength) {
                                        ++this.LastCommandRepeatCount;
                                    } else {
                                        this.LastCommandRepeatCount = 1;
                                        this.LastCommand = ReceivedCommand;
                                    }
                                    
                                    // Fire the event.
                                    this.OnSircsCommandReceived(new SircsCommandReceivedEventArgs(ReceivedCommand, this.LastCommandRepeatCount));
                                }
                                break;
                        }
                    }
                } else { // If the current signal level is low, we may assume that we've just timed a high pulse.
                    // If a high pulse is too long, cancel any incoming commands.
                    if (SecondsElapsed > SircsReceiver.IntraBitMaxLength) {
                        this.ReceivingCommand = false;
                        this.BitTimer.Reset();
                        this.BitTimer.Start();
                        this.LastPinChangedTime = 0;
                    }
                }
            }
        }

        #endregion

    }
}

Currently, the software reacts to input events by running through a list of scripts, passing the command ID, device ID and command length (in bits) to each until one of them returns zero (ie, success) to indicate that it has processed the button.

Scripts list
Scripts list

The advantage to this method is that the end-user could customise the behaviour of the software to their own liking very easily. For example, here's the PowerDVD.js file from above, which allows me to control PowerDVD from a PlayStation 2 DVD remote control:

// Table of commands.
var Commands = [
    { Command : 0x00, Device : 0x093A, Length : 20, Shortcut : '1' },         // 1
    { Command : 0x01, Device : 0x093A, Length : 20, Shortcut : '2' },         // 2
    { Command : 0x02, Device : 0x093A, Length : 20, Shortcut : '3' },         // 3
    { Command : 0x03, Device : 0x093A, Length : 20, Shortcut : '4' },         // 4
    { Command : 0x04, Device : 0x093A, Length : 20, Shortcut : '5' },         // 5
    { Command : 0x05, Device : 0x093A, Length : 20, Shortcut : '6' },         // 6
    { Command : 0x06, Device : 0x093A, Length : 20, Shortcut : '7' },         // 7
    { Command : 0x07, Device : 0x093A, Length : 20, Shortcut : '8' },         // 8
    { Command : 0x08, Device : 0x093A, Length : 20, Shortcut : '9' },         // 9
    { Command : 0x09, Device : 0x093A, Length : 20, Shortcut : '0' },         // 0
    { Command : 0x0B, Device : 0x093A, Length : 20, Shortcut : '{ENTER}' },   // Enter
    { Command : 0x0E, Device : 0x093A, Length : 20, Shortcut : '{ESC}' },     // Return
    { Command : 0x1A, Device : 0x093A, Length : 20, Shortcut : 'lt' },        // Title
    { Command : 0x2A, Device : 0x093A, Length : 20, Shortcut : 'x' },         // A<->B
    { Command : 0x28, Device : 0x093A, Length : 20, Shortcut : 'd' },         // Time
    { Command : 0x2C, Device : 0x093A, Length : 20, Shortcut : '^r' },        // Repeat
    { Command : 0x30, Device : 0x093A, Length : 20, Shortcut : 'p' },         // Previous
    { Command : 0x31, Device : 0x093A, Length : 20, Shortcut : 'n' },         // Next
    { Command : 0x32, Device : 0x093A, Length : 20, Shortcut : '{ENTER}' },   // Play
    { Command : 0x33, Device : 0x093A, Length : 20, Shortcut : 'b' },         // Scan <<
    { Command : 0x34, Device : 0x093A, Length : 20, Shortcut : 'f' },         // Scan >>
    { Command : 0x38, Device : 0x093A, Length : 20, Shortcut : 's' },         // Stop
    { Command : 0x39, Device : 0x093A, Length : 20, Shortcut : ' ' },         // Pause
    { Command : 0x54, Device : 0x093A, Length : 20, Shortcut : 'z' },         // Display
    { Command : 0x60, Device : 0x093A, Length : 20, Shortcut : '^b' },        // Slow <<
    { Command : 0x61, Device : 0x093A, Length : 20, Shortcut : 't' },         // Slow >>
    { Command : 0x63, Device : 0x093A, Length : 20, Shortcut : 'u' },         // Subtitle
    { Command : 0x64, Device : 0x093A, Length : 20, Shortcut : 'h' },         // Audio
    { Command : 0x65, Device : 0x093A, Length : 20, Shortcut : 'a' },         // Angle
    { Command : 0x79, Device : 0x093A, Length : 20, Shortcut : '{UP}' },      // Up
    { Command : 0x7A, Device : 0x093A, Length : 20, Shortcut : '{DOWN}' },    // Down
    { Command : 0x7B, Device : 0x093A, Length : 20, Shortcut : '{LEFT}' },    // Left
    { Command : 0x7C, Device : 0x093A, Length : 20, Shortcut : '{RIGHT}' },   // Right
];

// Search for the matching command.
var Command = null;
for (var enumerator = new Enumerator(Commands); !enumerator.atEnd(); enumerator.moveNext()) {
    var TestCommand = enumerator.item();
    if (TestCommand.Command == WScript.Arguments(1) && TestCommand.Device == WScript.Arguments(2) && TestCommand.Length == WScript.Arguments(3)) {
        Command = TestCommand;
        break;
    }
}

// No command.
if (!Command) WScript.Quit(1);

// Find the PowerDVD process ID.
var PowerDvdId = null;
var WmiService = GetObject('winmgmts://./root/cimv2');
var Processes = WmiService.ExecQuery('Select ProcessId From Win32_Process Where Name="PowerDVD.exe"');
for (var enumerator = new Enumerator(Processes); !enumerator.atEnd(); enumerator.moveNext()) {
    PowerDvdId = enumerator.item().ProcessId;
    break;
}

// If we haven't found the process ID, quit with an error.
if (!PowerDvdId) WScript.Quit(1);

// Activate the PowerDVD instance.
var WshShell = new ActiveXObject('WScript.Shell');
WshShell.AppActivate(PowerDvdId);

// Send the shortcut keys.
WshShell.SendKeys(Command.Shortcut);

WScript.Quit(0);

Unfortunately, this method has quite a lot of overhead. This becomes a problem when you consider that commands are repeated every 45ms. Currently I avoid the issue by not allowing any keys to repeat, but some keys - such as the volume keys - would need to repeat when held.

I'm unsure as the best path to take. One idea that has crossed my mind would be to set up each remote control you were going to use beforehand (though I suppose I could build up a database of remote controls and bundle them with the software). You could then set whether each key should repeat or not, and attach a meaningful string to each button. This would also allow for more protocols to be supported other than SIRCS, and you could set it up so that the Play button on a Sony remote control generated the string "play" and passed that to the script(s) as well as the Play button on a Panasonic or Toshiba remote control rather than juggling control codes.

Decoding SIRCS commands with a PIC16F84

Sunday, 1st March 2009

Some time ago I was working on a simple Z80-based computer. It has a PS/2 keyboard and mouse port for user input, and these are implemented using a large number of discrete parts - transistor drivers with all manner of supporting latches and buffers. The AT protocol (which the PS/2 keyboard and mouse inherit) is entirely implemented in software by the Z80.

On the one hand this design has a certain purity, but it ties the CPU up every time data is to be transferred. The keyboard sends data when it feels like it, so if you wished to perform some function based on a key press event you'd need to poll the port periodically, assuming that if communications time out there's no key waiting. All this hanging around does nothing good for performance.

As it turns out I found a PIC16F84 in an old school project over the weekend, so downloaded its datasheet and the MPLAB IDE and tried to puzzle it out.

The 16F84 is a pretty venerable microcontroller with a 1K flash memory for program code, 68 bytes of data RAM and 64 bytes of data EEPROM. It can run at up to 10MHz, and is based on a high-performance RISC CPU design. It has 13 digital I/O pins, each of which can be configured individually as either an input or an output. I'm well aware there are far better microcontrollers around these days, but this one was just sitting around doing nothing.

Above is the circuit I constructed to work with the 16F84. The HRM538BB5100 in the top-right is an infrared demodulator and amplifier module; it will output 5V until it receives a 38kHz infrared signal (such as the one emitted by most remote controls) at which point it outputs 0V. By timing the lengths of the IR pulses one could decode a remote control signal, and that's the aim of this project - decode a command from a Sony remote control and display it on the two 7-segment displays. The 10MHz crystal is probably overkill for this simple task, but it's the slowest I had available!

In fact, the 10MHz crystal works out quite neatly. Most instructions execute in one instruction cycle, which is four clock cycles. Four clock cycles at 10MHz is 400nS. The 16F84 has an internal timer that counts up after every instruction cycle and triggers an interrupt when it overflows from 255 back to 0; 400nS*256=102.4µs. If we call that 100µs (close enough for jazz) then it overflows 10 times every millisecond. The SIRCS protocol is based around multiples of 0.6ms, which makes this rate very easy to work with.

; ========================================================================== ;
; Pins:                                                                      ;
; RB0~RB6: Connected to A~G on the two seven-segment displays.               ;
; RB7:     Connected via a 220R resistor to cathode of the left display.     ;
;          Inverted and connected via a 220R resistor to right display's     ;
;          cathode.                                                          ;
; RA0:     Connected to the output of the HRM538BB5100.                      ;
; ========================================================================== ;

#include <p16F84.inc>

	list p=16F84

	__CONFIG   _CP_OFF & _WDT_OFF & _PWRTE_ON & _HS_OSC

; ========================================================================== ;
; Variables                                                                  ;
; ========================================================================== ;
	udata
IsrW       res 1 ; Temporary storage used to preserve state during the
IsrStatus  res 1 ; interrupt service routine.

Display    res 1 ; Value shown on 7-segment displays.

PulseTimer res 1 ; Counter to time the length of pulses.

BitCounter res 1 ; Number of bits being received.
Command    res 1 ; SIRCS command.

; ========================================================================== ;
; Reset                                                                      ;
; ========================================================================== ;
ResetVector code 0x0000
	goto Main

; ========================================================================== ;
; Interrupt Service Routine                                                  ;
; ========================================================================== ;
ISR code 0x0004

	; Preserve W and STATUS.
	movwf IsrW
	swapf STATUS,w
	movwf IsrStatus

	; Update value shown on two 7-segment displays.
	movfw Display
	btfsc PORTB,7
	swapf Display,w
	andlw h'F'
	call Get7SegBits
	btfss PORTB,7
	xorlw b'10000000'
	movwf PORTB

	; Increment pulse timer.
	incfsz PulseTimer,w
	movwf PulseTimer

	; Acknowledge timer interrupt.
	bcf INTCON,T0IF

	; Restore W and STATUS.
	swapf IsrStatus,w
	movwf STATUS
	swapf IsrW,f
	swapf IsrW,w	
	retfie

; ========================================================================== ;
; Times the length of a "low" pulse.                                         ;
; ========================================================================== ;
; Out: W - Length of pulse.                                                  ;
; ========================================================================== ;
TimeLow
	clrf PulseTimer
TimeLow.Wait
	btfsc PORTA,0
	goto TimeLow.GoneHigh
	incfsz PulseTimer,w
	goto TimeLow.Wait
TimeLow.GoneHigh
	movfw PulseTimer
	return

; ========================================================================== ;
; Times the length of a "high" pulse.                                        ;
; ========================================================================== ;
; Out: W - Length of pulse.                                                  ;
; ========================================================================== ;
TimeHigh
	clrf PulseTimer
TimeHigh.Wait
	btfss PORTA,0
	goto TimeHigh.GoneLow
	incfsz PulseTimer,w
	goto TimeHigh.Wait
TimeHigh.GoneLow
	movfw PulseTimer
	return

; ========================================================================== ;
; Convert a hex nybble (0-F) into a format that can be displayed on a 7-seg  ;
; display.                                                                   ;
; ========================================================================== ;
; In: W. Out: W.                                                             ;
; ========================================================================== ;
Get7SegBits
	addwf PCL, f
	dt b'00111111' ; 0
	dt b'00000110' ; 1
	dt b'01011011' ; 2
	dt b'01001111' ; 3
	dt b'01100110' ; 4
	dt b'01101101' ; 5
	dt b'01111101' ; 6
	dt b'00000111' ; 7
	dt b'01111111' ; 8
	dt b'01101111' ; 9
	dt b'01110111' ; A
	dt b'01111100' ; b
	dt b'00111001' ; C
	dt b'01011110' ; d
	dt b'01111001' ; E
	dt b'01110001' ; F

; ========================================================================== ;
; Start of the main program.                                                 ;
; ========================================================================== ;
Main

	; Set PORTB to be an output.
	bsf STATUS,RP0
	clrw
	movwf TRISB
	bcf STATUS,RP0

	; Configure TMR0.
	bsf STATUS,RP0
	bcf OPTION_REG,T0CS ; Use internal instruction counter.
	bcf STATUS,RP0

	; Enable TMR0 interrupt.
	bsf INTCON,T0IE
	bsf INTCON,GIE

	clrf Display

; ========================================================================== ;
; Main program loop.                                                         ;
; ========================================================================== ;
Loop

WaitCommand
	; Loop around waiting for a low to indicate incoming data.
	btfsc PORTA,0
	goto WaitCommand

	; Start bit (2.4mS).
	call TimeLow
	; Check that it's > 2mS long.
	sublw d'20'
	btfsc STATUS,C
	goto WaitCommand ; w<=20

	; Reset the command variable and get ready to read 7 bits.
	clrf Command
	movlw d'7'
	movwf BitCounter	

ReceiveBit
	; Time the pause; should be < 1mS.
	call TimeHigh
	sublw d'10'
	btfss STATUS,C
	goto WaitCommand

	; Time the input bit (0.6ms = low, 1.2ms = high).
	call TimeLow
	sublw d'9'
	; Shift into the command bit.
	rrf Command,f

	decfsz BitCounter,f
	goto ReceiveBit

	bsf STATUS,C
	rrf Command,f
	comf Command,f

	movfw Command
	movwf Display

	goto Loop

; ========================================================================== ;
; Fin.                                                                       ;
; ========================================================================== ;
	end

The final source code is above. I'm not sure how well-written it is, but it works; pointing a Sony remote control at the receiver and pressing a button changes the value shown on the seven-segment display. PICmicro assembly is going to get take a little getting used to; instructions are ordered "backwards" to the Intel order I'm used to (op source,destination instead of the more familiar op destination,source) and as far as I can tell literals default to being interpreted as hexadecimal as opposed to decimal.

With some luck I can now teach the 16F84 the AT protocol and replace a large number of parts on the Z80 computer project with a single IC. It does feel a little like cheating, though!

Expression Evaluation in Z80 Assembly

Tuesday, 24th February 2009

The expression evaluators I've written in the past have been memory hungry and complex. Reading the BBC BASIC ROM user's guide introduced me to the concept of expression evaluation using top-down analysis, which only uses a small amount of constant RAM and the stack.

I took some time out over the weekend to write an expression evaluator in Z80 assembly using this technique. It can take an expression in the form of a NUL-terminated string, like this:

.db "(-8>>2)+ceil(pi())+200E-2**sqrt(abs((~(2&4)>>>(30^sin(rad(90))))-(10>?1)))",0

and produce a single answer (or an error!) in the form of a floating-point number. The source code and some notes can be downloaded here.

I initially wrote a simple evaluator using 32-bit integers. I supported the operations the 8-bit Z80 could do relatively easily (addition, subtraction, shifts and logical operations) and got as far as 32-bit multiplication before deciding to use BBC BASIC's floating-point maths package instead. The downside is that BBC BASIC has to be installed (the program searches for the application and calls its FPP routine).

I'm not sure if the technique used is obvious (I'd never thought of it) but it works well enough and the Z80 code should be easy to follow - someone may find it useful. smile.gif

Nibbles and Logo

Thursday, 19th February 2009

Work on BBC BASIC has slowed down quite a bit, with only minor features being added. A *FONT command lets you output large or font sized text to the graphics cursor position regardless of the current MODE:

Mixed-Fonts.gif
10 MODE 3
20 VDU 5
30 MOVE 0,255 : PRINT "Small"
40 *FONT LARGE
50 MOVE 0,227 : PRINT "Large"
60 VDU 4
70 PRINT TAB(0,3) "Small (VDU 4)"

Another new command is the dangerous *GBUF that can - when used correctly - let you switch the location of the graphics buffer. You can simulate greyscale by quickly flickering between two different images on the LCD, which is where this command may come in use.

2009.02.11.01.gif

Snake/Nibbles is a fun game and an easy one to write, so here's a simple implementation that features variable speeds and mazes. The game runs quickly on a 6MHz TI-83+, which I'm happy with. And yes, I know I'm terrible at it. wink.gif

One thing I've always been pretty bad at is writing language parsers resulting in poor performance and bugs. I've started writing a primitive Logo interpreter in C# to try and improve my skills in this area. So far it supports a handful of the basic language features and statements:

- print [Hello World]
Hello World
- make "animals [cat dog sheep] show :animals
[cat dog sheep]
- make "animals lput "goat :animals show :animals
[cat dog sheep goat]
- print last :animals
goat
- repeat 2 [ print "A repeat 2 [ print "B ] ]
A
B
B
A
B
B
- show fput [1 2 3] [4 5 6]
[[1 2 3] 4 5 6]
- [10 9 8]
Not sure what to do with [10 9 8]

(No, no turtle graphics yet wink.gif). There's no support for infix operators yet. The BBC BASIC ROM manual describes the top-down parsing method it uses to evaluate expressions so I'm going to attempt to reimplement that.

One issue I've already run into are the parenthesis rules: for example the sum function outside parentheses only allows two arguments, but inside parentheses works until the closing parenthesis:

- print (sum 4 5)
9
- print (sum 4 5 6)
15
- print sum 4 5
9
- print sum 4 5 6
9
Not sure what to do with 6

I'm not sure whether a "this statement was preceded by an opening parenthesis" flag would be sufficient.

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