Maxim Integrated Secure Microcontroller - Manual (Page 170)

Secure Microcontroller User’s Guide
170 of 187
;***********************************
;
; This subroutine performs a “context switch: to the CE2 data ; space and
then reads one byte from the timekeeping device. ; Then it switches back to
the CE1 data space and returns ; the byte read in the accumulator, with all
other registers ; unchanged.
;
RBYTE: PUSH
DPL ;Save the data
PUSH DPH ; pointer on stack.
PUSH MCON ;Save MCON register.
ORL MCON,#4 ;Switch to CE2.
PUSH B ;Save the B register.
MOV DPL,#4 ;Set up for data input.
MOV DPH,#0 ;Set high address byte.
MOV B,#8 ;Set the bit count.
LI: PUSH ACC ;Save the accumulator.
MOVX A,@DPTR ;Input the data bit from the RTC into
; ACC.7.
RLC A ;Move it to carry.
POP ACC ;Get the accumulator.
RRC A ;Save the data bit.
DJNZ B,LI ;Loop for a whole byte.
POP B ;Restore the B register.
POP MCON ;Restore the MCON register.
POP DPH ;Restore the data
POP DPL ; pointer from stack.
RET ;Return.
;
;************************************
;*** SUBROUTINE TO WRITE A DATA BYTE
;************************************
;
; This subroutine performs a “context switch” to the CE2 data
; space and then writes one byte from the accumulator to the
; timekeeping device. Then it switches back to the CE1 data
; space and returns with all registers unchanged.
;
WBYTE: PUSH
DPL ;Save the data
PUSH DPH ; pointer on stack.
PUSH MCON ;Save the MCON register.
ORL MCON,#4 ;Switch to CE2.
PUSH B ;Save the B register.
MOV DPH,#0 ;Set high address byte.
MOV B, #8 ;Set the bit Count.
LO: PUSH ACC ;Save the accumulator.
ANL A, #1 ;Set up bit for output.
MOV DPL,A ;Set address to write bit.
MOVX A, @DPTR ;Output the data bit.
POP ACC ;Restore the accumulator.
RR A ;Position next bit.
DJNZ B, LO ;Loop for a whole byte.
POP B ;Restore the B register.
POP MCON ;Restore the MCON register.
POP DPH ;Restore the data
POP DPL ; pointer from stack.
RET ;Return.
;
END ;End of program.