在一些应用中，经常要用到双串口，但是一般单片机只提供一个串口，其实完全可以用普通I/O口模拟一个串口。以下的程序是我编写的模拟串口程序，程序中使用了单片机的定时器0，和INT0中断。数据的发送和接收由中断程序自动进行。程序已经过AVR仿真器仿真和实际烧片使用，证明可靠。有一点需要说明的是，此模拟的串口为半双工方式。
   主程序中，单片机将标准串口设置为115200bps,将模拟串口设置为19200bps。单片机将标准串口收到的数据从模拟串口发送出去，将模拟串口接收到的数据从标准串口发送回来。

;**************************************************************************************************
;*title:halfduplexuartsimulatonprogram
;*version:1.0
;*programtime:2001/11/05
;*target:AT90S8515
;*design:zsmbj@beijing
;**************************************************************************************************
.include"c:\programfiles\atmel\avrstudio\appnotes\8515def.inc"
;BPS=19200
;F=11059200

.equN=72

.equtxd0=3;uart0txd
.equrxd0=2;uart0rxd
;****************************************************************
.equstack=0x0ff
;****************************************************************
;bitdefine
.equrdr=0
.equfe0=1
.equtd=6
.equbusy=7
;registerdefine
.deftemp=r16
.defsbuf0=r17
.defstatus=r18
.defbit_cnt=r19
;**************************************************************************************************
.org0x00
rjmpreset
.org0x01
rjmpint00
.org0x07
rjmptimer0_int
;**********************************************************
.cseg
;**********************************************************
;****initial
;**********************************************************
.org0x0010
;resetat90s8515
reset:
lditemp,0b00001000
outddrb,temp

lditemp,high(stack);stack
outsph,temp
lditemp,low(stack)
outspl,temp

lditemp,5;baud115200bpsat11.0592Mfosc
outubrr,temp
lditemp,0b00011000;enablerxandtx
outucr,temp
;timer0set
lditemp,0x02;ck/80.72338us
outtccr0,temp

lditemp,0x0a;disableoutsidesram,int0falledgemakeainterrupt
outmcucr,temp
lditemp,0x40
outgimsk,temp;enableint0andint1interrupt

lditemp,0
movstatus,temp
sbiportb,txd0;txd0bit=1

sei;globleinterruptenable
rjmpmain
;******************************************
timer0_int:
pushtemp
intemp,sreg
pushtemp

lditemp,(256-N)
outTCNT0,temp
incbit_cnt

sbrsstatus,td
rjmptimer0_receive
;>>>>>>>>>>>>>>>>>>>>>>>>>>
;senddata8databitand1stopbit
timer0_send:
sbrcbit_cnt,3;ifbit_cnt=8thenstopbit
rjmptimer0_send_stop
timer0_send_data:
sbrcsbuf0,0;txd=0
sbiportb,txd0
sbrssbuf0,0;txd=1
cbiportb,txd0
lsrsbuf0
rjmptimer0_end
timer0_send_stop:
sbiportb,txd0;stopbit=1
sbrcbit_cnt,0
rjmptimer0_complete;ifbit_cnt=9thencomplete
;;;;;;;;;;;;;;;;;;;
intemp,gifr
sbrtemp,(1<outgifr,temp;clrint0flag

intemp,gimsk
sbrtemp,(1<outgimsk,temp;enablegimsk/int0

rjmptimer0_end
;>>>>>>>>>>>>>>>>>>>>>>>>>>
;receivestart1bitdata8bitstop1bit
timer0_receive:
cpibit_cnt,1;ifbit_cnt=1thenstartbit
breqtimer0_receive_start
cpibit_cnt,10;ifbit_cnt=10thenstopbit
breqtimer0_receive_stop

rjmptimer0_receive_data
timer0_receive_start:
sbispind,rxd0
rjmptimer0_end

cbrstatus,(1<rjmptimer0_complete
timer0_receive_data:
sec
sbispind,rxd0;getrxd0data
clc
rorsbuf0
rjmptimer0_end
timer0_receive_stop:
cbrstatus,(1<sbispind,rxd0
rjmptimer0_complete
sbrstatus,(1<sbrstatus,(1<;>>>>>>>>>>>>>>>>>>>>>>>>>>
timer0_complete:
intemp,timsk
cbrtemp,(1<outtimsk,temp;disabletimsk/toie0
;;;;;;;;;;;;;;;;;;;
intemp,gifr
sbrtemp,(1<outgifr,temp;clrint0flag

intemp,gimsk
sbrtemp,(1<outgimsk,temp;enablegimsk/int0

cbrstatus,(1<poptemp
outsreg,temp
poptemp

reti
;******************************************
int00:
pushtemp
intemp,sreg
pushtemp

lditemp,(256-N/2);skip0.5bit
outTCNT0,temp

ldistatus,(1<clrbit_cnt

intemp,tifr
sbrtemp,(1<outtifr,temp;clrtifr/tov0

intemp,timsk
sbrtemp,(1<outtimsk,temp;enabletimsk/toie0

intemp,gimsk
cbrtemp,(1<outgimsk,temp;disablegimsk/int0

poptemp
outsreg,temp
poptemp
reti
;**********************************************************rxd0_data:
txd0_data:
ldistatus,(1<pushtemp
intemp,gimsk
cbrtemp,(1<outgimsk,temp;disablegimsk/int0
poptemp

serbit_cnt;bit_cnt=0xff
movsbuf0,temp;senddata

lditemp,(256-N)
outTCNT0,temp;wait1bittimer0interrupt

intemp,tifr
sbrtemp,(1<outtifr,temp;clrtifr/tov0

intemp,timsk
sbrtemp,(1<outtimsk,temp;enabletimsk/toie0

cbiportb,txd0;uartstart

ret
;******************************************
rxd0_data:
sbrsstatus,fe0;iffe0=0thenexit
rjmprxd0_data_end
cbrstatus,(1<movtemp,sbuf0
rxd0_data_end:
ret
;******************************************

;uartreceivedabytsfromuartandthenreturnitfromuart0:
;uartreceivedabytsfromuart0andthenreturnitfromuart:
main:
sbicusr,rxc
rjmpsend_115200

sbrsstatus,rdr
rjmpuart_end
send_19200:
rcallrxd0_data;getuartdatafrom19200bpsuart0

wait2:sbisusr,udrie
rjmpwait2
outudr,temp;senddatato115200bpsuart
rjmpuart_end

send_115200:
intemp,udr;getuartdatafrom115200bpsuart
sbicusr,fe
rjmpuart_end;iffeerrthenend

wait3:sbrcstatus,td;waitsendflag
rjmpwait3
rcalltxd0_data;senddatato19200bpsuart0
uart_end:
rjmpmain
;**********************************************************
.exit
;**********************************************************

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