.file "main.S"

#include <msp430g2553.h>


#define PC r0
#define SP r1
#define SR r2

;; --- register usage --------------------------------
;; r4: synchronization between main loop and isr
;;     bit0 signals byte done from isr
;; r5: data byte to be handled by isr
;; r6: bit-counter, only used within isr
;; r7: screen data start/next pointer
;; r8: screen data end pointer
;; r9: next screen data byte, prepared in mainloop

#define SIGNAL_INIT_VALUE 0x00
#define SIGNAL_BYTE_DONE 0x01
#define SIGNAL_ISR_ENABLE 0x02
#define SIGNAL_REGISTER r4

#define DATA_BYTE_REGISTER r5
#define NEXT_DATA_BYTE_REGISTER r9
#define DATA_NEXT_ADDRESS_REGISTER r7
#define DATA_LAST_ADDRESS_REGISTER r8

#define BIT_COUNTER_REGISTER r6
#define BIT_COUNTER_INIT_VALUE 0x01
;; ---------------------------------------------------


  .section ".rodata","a"
screendata:
  .byte   0b00000001
  .byte   0b00000011
  .byte   0b00000111
  .byte   0b00001111
screendataend:
  .byte   0xff
red:
colors1:
  .byte   0b01001111
colors2:
  .byte   0b01000111
colors3:
  .byte   0b01000011
  .byte   0
blue:
  .byte   0b11001111
  .byte   0b11000111
  .byte   0b11000011
  .byte   0
green:
  .byte   0b10001111
  .byte   0b10000111
  .byte   0b10000011
  .byte   0

  ;; .text is the name of the section, it is a hint for the linker to
  ;; allocate the section
  ;; ax: a means allocatable by linker, x means executable
  ;; @progbits is a hint for the linker to allocate this section into
  ;; program memory (flash)
  .section ".text","ax",@progbits
_start:
  ;; disable watchdog
  mov.w   #WDTPW|WDTHOLD, &WDTCTL

  ;; configure clock system to the highest frequency
  mov.b   #DCO0|DCO1|DCO2, &DCOCTL
  mov.b   #XT2OFF|RSEL0|RSEL1|RSEL2|RSEL3, &BCSCTL1
  mov.b   #0, &BCSCTL2
  mov.b   #0, &BCSCTL3

  ;; initialize stack pointer with value from linker
  mov.w   #__stack, SP

init:
  ;; configuration of GPIO Ports
  mov.b   #BIT0|BIT1|BIT2, &P1DIR
  mov.b   #BIT1|BIT4, &P2DIR
  mov.b   #BIT1|BIT4, &P2SEL

  ;; timer configuration
  ;;  configure and stop timer
  ;;  cycle time is 56.25ns
  mov.w   #ID_0|MC_0|TACLR|TASSEL_2, &TA1CTL
  ;;  2.0us 
  mov.w   #45, &TA1CCR0
  ;;  a bit less
  mov.w   #10, &TA1CCR1
  mov.w   #22, &TA1CCR2
  ;;  configure output mode for TA0.1
  mov.w   #CCIE, &TA1CCTL0
  mov.w   #OUTMOD_7, &TA1CCTL1
  mov.w   #OUTMOD_7, &TA1CCTL2

  ;; initialize bit-counter for isr
  mov.b   #BIT_COUNTER_INIT_VALUE, BIT_COUNTER_REGISTER
  ;; initialize isr-sync register
  mov.b   #SIGNAL_INIT_VALUE, SIGNAL_REGISTER

  ;; screen data start/next into DATA_NEXT_ADDRESS_REGISTER
  mov.w   #screendata, DATA_NEXT_ADDRESS_REGISTER
  ;; screen data end into DATA_LAST_ADDRESS_REGISTER
  mov.w   #screendataend, DATA_LAST_ADDRESS_REGISTER
  ;; load first screen data value into DATA_BYTE_REGISTER
  mov.b   @DATA_NEXT_ADDRESS_REGISTER, DATA_BYTE_REGISTER
  inc.w   DATA_NEXT_ADDRESS_REGISTER

  ;; start timer in up mode
  bis.w   #MC0, &TA1CTL
  ;; enable interrupts
  eint


mainloop:
  ;; prepare next byte to handle by isr
  ;; first, check whether we are already at the end of screen data
  cmp.w   DATA_NEXT_ADDRESS_REGISTER, DATA_LAST_ADDRESS_REGISTER
  jnz     mainloop_prepare_next_byte
  ;; in that case rollover to the start again
  mov.w   #screendata, DATA_NEXT_ADDRESS_REGISTER
mainloop_prepare_next_byte:
  ;; load the byte from the screen data next pointer
  mov.b   @DATA_NEXT_ADDRESS_REGISTER, NEXT_DATA_BYTE_REGISTER
  ;; and increase the pointer
  inc.w   DATA_NEXT_ADDRESS_REGISTER

mainloop_wait_for_isr:
  ;; check bit0 in sync register, wait for the signal from 
  ;; the isr
  bit     #SIGNAL_BYTE_DONE, SIGNAL_REGISTER
  jz      mainloop_wait_for_isr

  ;; load data
  mov.b   NEXT_DATA_BYTE_REGISTER, DATA_BYTE_REGISTER
  ;; clear the signal
  bic     #SIGNAL_BYTE_DONE, SIGNAL_REGISTER
  ;; mark the cycle
  bis     #BIT2, &P1OUT
  bic     #BIT2, &P1OUT

  ;; continue
  jmp     mainloop


; --- timer isr ---
  ;; r6: exclusively used by isr as bit-counter
timer1_a0_isr:
  ;; func begin marker
  bis     #BIT0, &P1OUT
  ;; check isr idle bit
  bit     #SIGNAL_ISR_ENABLE, SIGNAL_REGISTER
  jnz     timer1_a0_isr_exit

  ;; shift msb of data register r5 into carry flag and set or reset P1.1 accordingly
  rla.b   DATA_BYTE_REGISTER
  jnc     timer1_a0_isr_false_bit
  bis     #BIT1, &P1OUT
  jmp     timer1_a0_isr_end
timer1_a0_isr_false_bit:
  bic     #BIT1, &P1OUT
   
  ;; shift bit-counter, after eight shifts signal byte done and reset bit-counter
timer1_a0_isr_end:
  rla.b   BIT_COUNTER_REGISTER
  jnc     timer1_a0_isr_exit

  ;; reset bit-counter
  mov.b   #BIT_COUNTER_INIT_VALUE, BIT_COUNTER_REGISTER
  ;; signal byte done
  bis     #SIGNAL_BYTE_DONE, SIGNAL_REGISTER

timer1_a0_isr_exit:
  ;; func end marker
  bic     #BIT0, &P1OUT
  reti



  ;; .resetvec comes from linker
  .section ".resetvec","ax",@progbits
  .word   _start

  .end