di2616.txt 
;**************************************************************************************************
; LISTING 1 - FREQUENCY-COUNTER ROUTINE FOR 89C2051
;
; "Microcontroller makes effective frequency counter," EDN, Nov 23, 2000, pg 164

; http://www.ednmag.com/ednmag/reg/2000/11232000/designideas.htm#24di6
;
***************************************************************************************************

; Program counts the pulses and transmits through p3.1 pin only without handshaking signal.

        .Equ Frqflg,0fh                 ;use a bit flag to signal main program
        .Equ Baudnum,0f3h               ;number loaded in TH1 for 2403.8 baud
Freq:
        .Org 0000h
         Mov SP, #30h                   ;set stack above register/bit area
         Sjmp over                      ;jump over T1 interrupt address

        .Org 000bh
         Inc r2
         Reti

        .Org 001bh                     ;T1 overflow flag interrupt to here
         Setb psw.3                    ;swich to register bank1
         Inc r0                        ;count R0 up until overflow at 00h
         Cjne r0, #00h,Checktime       ;check to see if time is up
         Inc r1                        ;or inc R1 when R0 rolls over
Checktime:
        Cjne r1, #27h, Goback          ;check R1 for terminal count
        Cjne r0, #10h, Goback           ;check R0 for terminal count
        Clr tr0                        ;stop T0
        Clr tr1                        ;stop T1 time before T0 stopped =
        Setb Frqflg                    ;signal main program that T0 =freq.
Goback:
        Clr psw.3                      ;return to bank 0 registers
        Reti                           ;return to main program

Over:
        Mov tcon, #00h                 ;all timers stopped - flags reset
        Set psw.3                      ;select register bank 1 and reset R0, R1
        Mov r0, #00h
        Mov r1, #00h
        Clr psw.3                      ;return to bank 0
        Mov tmod, #25h                 ;T1 a mode 2 timer, T0 mode 1 counter
        Mov tl1, #9ch                  ;start TL1 at 9ch
        Mov th1, #9ch                  ;TH1 = 156d, overflows in 100 clocks
        Mov tl0, #00h                  ;zero T0
        Mov th0, #00h
        Clr Frqflg                     ;reset the freq. measured flag
        Mov tcon, #50h                 ;start timer T1 and counter T0
        Mov ie, #88h                   ;enable global and T1 overflow interrupts

Simulate:
        Jbc Frqflg, Getfrq             ;have main program test "frqflg"
        Sjmp Simulate                  ;loop here until freq.is measured

Getfrq:
        Anl pcon, #7fh                 ;set SMOD bit to 0 for Baud * 32 rate          
        Anl tmod, #30h                 ;alter timer T1 configuration only
        Orl tmod, #20h                 ;set timer T1 as an 8 bit autoload
        Mov th1, #Baudnum               
        Setb tr1                       ;run T1
        Mov scon, #40h
        Mov sbuf, #'L'


Wait:
        Jbc ti, Next
        Sjmp Wait
Next:
        Mov sbuf, r2                   ;set UART to mode1
Wait1:
        Jbc ti, Next1
        Sjmp Wait1
Next1:
        Mov sbuf, th0                  ;transmit content of the th0
Wait2:
        Jbc ti, Next2                  ;wait for T1set before next transmission
        Sjmp Wait2                     ;else poll flag again
Next2:
        Mov sbuf, tl0                  ;now transmit tl0
                                       ;and wait until the ti flag is set
Wait3:
        Jbc ti, Over
        Sjmp Wait3
        
       . End