Datasheet

Synchronous Character Transmission Using the SPI
MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor 193
; ----------------------------------------------------------------------
;* SUBROUTINE INIT:
; ----------------------------------------------------------------------
INIT:
BSET PORTS,#$80 ; SET SS Line High to prevent glitch
MOVB #$E0,DDRS ; Configure PORT S input/ouput levels
; ; MOSI, SCK, SS* = ouput, MISO=Input
MOVB #$07,SP0BR ; Select serial clock baud rate < 100 KHz
MOVB #$12,SP0CR1 ; Configure SPI(SP0CR1): No SPI interrupts,
; ; MSTR=1, CPOL=0, CPHA=0
MOVB #$08,SP0CR2 ; Config. PORTS output drivers to operate normally,
; ; and with active pull-up devices.
LDX #DATA ; Use X register as pointer to first character
LDAA SP0SR ; 1st step to clear SPIF Flag, Read SP0SR
LDAA SP0DR ; 2nd step to clear SPIF Flag, Access SP0DR
BSET SP0CR1,#$40 ; Enable the SPI (SPE=1)
RTS ; Return from subroutine
; ----------------------------------------------------------------------
;* TRANSMIT SUBROUTINE
; ----------------------------------------------------------------------
TRANSMIT:
LDAA 1,X+ ; Load Acc. with "NEW" character to send, Inc X
BEQ DONE ; Detect if last character(0) has been transmitted
; ; If last char. branch to DONE, else
BCLR PORTS,#$80 ; Assert SS Line to start X-misssion.
STAA SP0DR ; Load Data into Data Reg.,X-mit.
; ; it is also the 2nd step to clear SPIF flag.
FLAG: BRCLR SP0SR,#$80,FLAG ;Wait for flag.
BSET PORTS,#$80 ; Disassert SS Line.
BRA TRANSMIT ; Continue sending characters, Branch to TRANSMIT.
DONE: RTS ; Return from subroutine
; ----------------------------------------------------------------------
; TABLE OF DATA TO BE TRANSMITTED
; ----------------------------------------------------------------------
DATA: DC.B 'Freescale'
DC.B $0D,$0A ; Return (cr) ,Line Feed (LF)
EOT: DC.B $00 ; Byte used to test end of data = EOT
END ; End of program