heart.mu4

| This file is part of muforth: https://muforth.dev/
|
| Copyright 2002-2024 David Frech. (Read the LICENSE for details.)

loading AVR heartbeat (software PWM)

assembler
| Helpful macro for waking tasks.
: wake    ( 'user)  asm{  \f dup  xl lds  xl dec  xl sts  } ;

| If a task is defined, wake it; otherwise, do nothing.
: ?wake   .equates. token' if  execute  \a wake  ^  then  2drop ( token) ;
forth

__meta
hex

| Output to modulate - probably connected to an LED.
| Here is an example; do something similar before loading this file.
|   0 PORTB ( bit port) 2constant heart-led

| We want the heartbeat to be a triangle wave. The simplest way to achieve
| this, with the fewest glitches and corner cases, is to XOR a sawtooth
| wave at frequency f with a polarity square wave at f/2.

| 00 01 02 ... fc fd fe ff 00 01 02 ... fc fd fe ff     Sawtooth
| 00 00 00 ... 00 00 00 00 ff ff ff ... ff ff ff ff     Polarity
| 00 01 02 ... fc fd fe ff ff fe fd ... 03 02 01 00     Output

1 var heart-beat        | how far we are into our 256-point pwm cycle
1 var heart-sawtooth    | current sawtooth value
1 var heart-polarity    | current polarity value - 00 or ff
1 var heart-output      | current output pwm = sawtooth xor polarity

| heart-beat is incremented by the timer overflow interrupt.
|
| heart-sawtooth, heart-polarity, and heart-output are re-computed by the
| timer task every time it is woken by the timer interrupt.

| At 16M clock, timer overflows at 62.5k, or every 16us.
| At 8M clock, timer overflows at 31.25k, or every 32us.
|
| To keep things running at the same rate in both cases, we increment
| heart-beat by 2 when running at 8M.

TIMER0_OVF handler
label timer0-overflow-handler
   x pushw  SREG xl in  xl push  ( push X and SREG)
   heart-beat xl lds  xl xh mov  xl inc
.ifndef cksel  ( using internal 8M RC clock)  xl inc  .then
   xl xh eor  heart-beat xl sts  ( xh has bits that have toggled)

   xl tst  0= if
      ( Wake the heart task every 256 ticks - every 4.096 ms.)
      ?wake heart-user-area
   then
   4 xh sbrs  never  ( jump to then unless bit 4 set)
      ( Wake button tasks every 16 ticks - every 256 us.)
      ?wake button0-user-area
      ?wake button1-user-area
   then

   | If heart-beat u< heart-output, light the LED.
   heart-beat xl lds  heart-output xh lds  xh xl cp ( beat - output)
   u< if  heart-led cbi  ( LED on)   else
          heart-led sbi  ( LED off)  then

   ( Restore SREG and X and exit handler)
   xl pop  SREG xl out  x popw  reti  ;c

| Consume one wakeup and call yield.
label nap
   u y movw  cli  0 g0 ldu ( status)  g0 inc  0 g0 stu  sei  yield jmp  ;c

label heart-task
   | Initialize everything.
   heart-led sbi  ( LED off)
   heart-led DDRB PORTB - + ( convert PORTx to DDRx) sbi  ( set pin as output)

   01 xl ldi  TIMSK0 xl out  ( enable TOVR0)
              TCCR0B xl out  ( set timer0 clock to CPU/1)
   00 xl ldi  TCCR0A xl out  ( set up timer0 - normal mode)
   ( leave xl set to 00!)

   | Zero everything.
   heart-beat xl sts
   heart-sawtooth xl sts
   heart-polarity xl sts
   heart-output xl sts

   | Enable watchdog.
   18 ( WDCE + WDE) h0 ldi
   8 ( WDE) 3 ( 16K = 0.125s timeout) + h1 ldi
   wdr  WDTCSR h0 out  WDTCSR h1 out

   | The heart of the task. ;-)
   begin
      wdr  nap rcall
      heart-polarity xh lds
      heart-sawtooth xl lds  xl inc  heart-sawtooth xl sts
      0= if  ( sawtooth rolled over)
         xh com  heart-polarity xh sts  ( complement polarity)
      then
      xh xl eor  heart-output xl sts  ( new output trigger point)
   again  ;c