© 2001 Ubicom, Inc. All rights reserved. - 6 - www.ubicom.com AN38 Asynchronous Transmitter & Receiver (UART) Virtual Peripheral Implementation ;*******************************************************************************
; VP: VP Multitasker
; ISR TASKS
;*******************************************************************************
IsrThread1                                                                                                    ; Serviced at ISR rate/4
;---------------------------VP: RS232 Transmit----------------------------------
;*******************************************************************************;
; Virtual Peripheral: Universal Asynchronous Receiver Transmitter (UART) These routines
; send and receive RS232 serial data, and are currently; configured (though modifications
; can be made for the popular "No parity-checking, 8 data bit, 1 stop bit" (N,8,1) data format. ; RECEIVING: The rs232Rxflag is set high whenever a valid byte of data has been received
; and it is the calling routine’s responsibility to reset this flag once the incoming
; data has been collected. ; TRANSMITTING : The transmit routine requires the data to be inverted and loaded
; (rs232Txhigh+rs232Txlow) register pair (with the inverted 8 data bits stored in rs232Txhigh
; and rs232Txlow bit 7 set high to act as a start bit). Then the number of bits ready for
; transmission (10=1 ;start + 8 data + 1 stop) must be loaded into the rs232Txcount register.
; As soon as this latter is ;done, the transmit routine immediately begins sending the data.
; This routine has a varying ;execution rate and therefore should always be placed after any
; timing-critical virtual peripherals ;such as timers, adcs, pwms, etc. Note:
;
; The transmit and receive routines are independent and either may be  removed, if not needed,
; to ;reduce execution time and memory usage, as long as the initial "BANK serial" (common)
; instruction is kept.  
; Input variable(s) : rs232Txlow (only high bit used), rs232Txhigh, rs232Txcount .
; output variable(s) : rs232Rxflag, rs232Rxbyte
; variable(s) affected : rs232Txdivide, rs232Rxdivide, rs232Rxcount
; Flag(s) affected : rs232Rxflag
; Program cycles: 17 worst case
; Variable Length?  Yes.
;*******************************************************************************
rs232Transmit _bank rs232TxBank ;2 switch to serial register bank decsz rs232TxDivide ;1 only execute the transmit routine jmp :rs232TxOut ;1 mov w,#UARTTxDivide ;1 load UART baud rate (50MHz) mov rs232TxDivide,w ;1   test rs232TxCount ;1 are we sending? snz ;1   jmp :rs232TxOut ;1 :txbitclc ;1 yes, ready stop bit rr rs232TxHigh ;1 and shift to next bit rr rs232TxLow ;1   dec rs232TxCount ;1 decrement bit counter snb rs232TxLow.6 ;1 output next bit clrb rs232TxPin ;1   sb rs232TxLow.6 ;1   setb rs232TxPin ;1,17   :rs232TxOut