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High speed RS232 for PIC

Jettisoning the ballast

Bits must be read at 6 11 15 19 24 28 32 37 cycles since start bit falling edge, which means at 3 8 … 34 instruction cycle of the routine. To read bits we will use the following instructions:

  btfsc RX ; test port
  bsf INDF,<i> ; set bit
  nop   ; delay
  nop   ; delay

As everyone may see, among 37 instructions spent on byte reading, only 16 are "productive", the others are just ballast, which we may replace with something useful, like saving context, incrementing byte counter, etc. It is important to notice, that bit-reading instructions do not affect STATUS so we may save it later, just before incrementing FSR or the counter. On the other hand, we cannot modify INDF before we ensure that the interrupt is caused by RX pin and cannot clear working file (INDF) with clrf before STATUS is saved. The following code sample solves all these issues:

IV   CODE 0x04

  movwf TMP_WREG ; 4: save W

V    CODE 0x05

  movlw 0 ; 5: safely clear W
  btfsc RX ; 6: test port RX
  movlw 1 ; 7: set bit 0
  btfss INTCON, INTF ; 8: now we have some time to look what caused this interrupt
  goto not_INT ; 9: not INT
  movwf INDF ; 10: save what already recived to buffer (clrf replacement)
  btfsc RX ; 11: test port
  bsf INDF,1 ; 12: set bit 1
  nope   ; 13, 14
  btfsc RX ; 15: test port
  bsf INDF,2 ; 16: set bit 2
  swapf STATUS, W ; 17: load STATUS
  movwf TMP_STATUS ; 18: save STATUS
  btfsc RX ; 19: test port
  bsf INDF,3 ; 20: set bit
  incf RS_COUNTER, F ; 21: already safe to increment counter
  movf TMP_WREG, W ; 22: W is not used we may restore WREG
  nop   ; 23:
  btfsc RX ; 24: test port
  bsf INDF,4 ; 25: set bit
  nope   ; 26, 27 still have plenty of time
  btfsc RX ; 28: test port
  bsf INDF,5 ; 29: set bit
  nope   ; 30, 31
  btfsc RX ; 32: test port
  bsf INDF,6 ; 33: set bit
  nope   ; 34, 35
  nop   ; 36
  btfsc RX ; 37: test port
  bsf INDF,7 ; 38: set the last bit
  incf FSR, F ; 39: advance current position, Z is discarded with 0 (FSR>0)
  btfsc TMP_STATUS, (Z+.4) ; 40: what was the last saved value of Z?
  setz   ; 41: restore Z, it was the only flag discarded
  bcf INTCON, INTF ; 42: interrupt handled
  retfie   ; 43: leave routine - 4 cycles of two stop bits left !!!

not_INT
     ; other handlers

Is it going to work? The simulator shows the following picture, where red points denote bit boundaries and green dashes – read instructions. Because of MP LAB SIM limitations, bit boundaries are aligned to the closest IC boundary and three extra nops added to routine to simulate hardware latency.
Figure 4. Reading at 230400
Figure4

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