Nuts & Volts - May 2007

of the compiler’s __PARAMx variables, we can use the NOPRESERVE keyword so that those variables — which aren’t changed during the ISR — are not saved and restored; this just wastes time if we don’t need it to happen.

The first VP encountered decrements the delay timer if it’s running. Remember, PAUSE will not work properly with the ISR so we’ll have to use a custom subroutine to handle that — we’ve done that in the last couple projects so I’m sure it’s old hat by now.

Next up is the sync timer. It will be incremented each time through the ISR and if it goes past the required idle time bit count (20), a flag will be set that will allow the foreground process to receive serial data. What you’ll see up ahead is that this flag and its control timer get cleared at the end of a received byte; this forces the program to ignore serial data for two byte periods after the last packet byte has been received.

And now for the receive UART. We’ve actually used this code a couple times before (see the serial seven-segment display project from January 05 for a detailed description). Briefly, the receive line is sampled until a start bit is detected. When this happens, the receive bit counter is loaded and the timer decremented. When the start bit timer (which is 1.5 bits long so that sampling starts in the middle of the first bit) expires, the line is

sampled and the bit timer reloaded with one bit period. After all bits have been received, the rxReady flag bit is set and the armed flag and packet sync timer are reset.

Receive:
  ASM
  JB    rxReady, RX_Done
  BANK serial
  MOVB  C, RX
  TEST rxCount
  JNZ  RX_Bit
  MOV   W, #9
  SC
  MOV   rxCount, W
  MOV   rxDivide, #Baud1x5

RX_Bit:
  DJNZ  rxDivide, RX_Done
  MOV   rxDivide, #Baud1x0
  DEC  rxCount
  SZ
  RR   rxByte
  JNZ  RX_Done
  SETB rxReady
  CLRB armed
  CLR  syncTimer

RX_Done:
  BANK 0
  ENDASM

■ FIGURE 2. Outputs schematic.

For those who might be new to SX/B, you can see how easily assembly code is integrated into the program through the ASM..ENDASM block. And to be honest, I didn’t create this code; I “liberated” it from Al Williams’ SX programming book Exploring the SX Microcontroller (available as a free PDF from Parallax). As good as SX/B is, there will be times when inserting some assembly code will be the best development choice — it’s nice that SX/B makes it so easy. Another great source of assembly routines is Günther Daubach’s excellent book Programming the SX Microcontroller. And don’t forget James Newton’s SX List ( www.sxlist.com).