I want to use the SPI on an Arduino Mega2560. To keep it simple I am trying to drive a74HC595 shift register. The SPI code is that in the Mega 2560 datasheet and also as I found on-line.
; SPItest3.asm Created: 29/11/2019
;*****************************************************************************
.include "./m2560def.inc"
.equ SS_pin = $0
.equ SCK_pin = $1
.equ MOSI_pin = $2
.equ test_pin = $4
.org 0x0000
rjmp reset
.org 0x002E
TIM0isr: ; timer 0 ISR
inc r3 ; add 1 to the count
reti
reset:
clr r1 ; set the SREG to 0
out SREG, r1
ldi r28, LOW(RAMEND) ; init the stack pointer to point to RAMEND
ldi r29, HIGH(RAMEND)
out SPL, r28
out SPH, r29
;---------------< Timer/Counter 0 >---------------
ldi r16, 1 ; set the Clock Selector Bits to 001, this puts Timer Counter 0
out TCCR0B, r16 ; TCNT0 in to FCPU/256 mode so it ticks at the CPU freq/256
ldi r16, (1<<TOIE0) ; set the Timer Overflow Interrupt Enable (TOIE0) bit
sts TIMSK0, r16 ; of the Timer Interrupt Mask Register (TIMSK0)
;---------------< SPI as Master >---------------
ldi r17, (1<<PB4)|(1<<PB2)|(1<PB1)|(1<<PB0) ; test, MOSI, SCK & SS are outputs
out DDRB, r17 ; Rest are inputs
ldi r17, (1<<SPE)|(1<MSTR)|(1<SPR1)|(1<SPR0) ; Enable SPI, Master, Clock = Fosc/128
out SPCR, r17
sei
main:
sbi PORTB,test_pin ; test HIGH \
ldi r21,$2 ; | Make pulse to see on oscilloscope
rcall delay ; |
cbi PORTB,test_pin ; test LOW /
ldi r21,$3
rcall delay
inc r18 ; something to send
cbi PORTB,SS_pin ; SS low at start of SPI
rcall SPI_Transmit
ldi r21,$15
rcall delay
sbi PORTB,SS_pin ; SS high at end of SPI, Load data to output of 74HC595
ldi r21,$3
rcall delay
rjmp main
SPI_Transmit: ;-------< Transmit SPI as Master >------
out SPDR, r18
in r17,SPSR
SPI_wait:
in r17,SPSR
sbrs r17,SPIF
rjmp SPI_wait
ret
delay: ;----------< Simple delay function >--------
clr r3 ; set count to 0
count:
mov r16, r3 ; test count
cp r16, r21
brne count ; continue if not equal
ret
There is no activity on either the MOSI, SCK pins or SS pin. If I remove the loop waiting for the transmission to end then the SS pin behaves as shown below.
The top blue trace is from a separate output (PORTB, 4) to give a trigger for the oscilloscope and the bottom red trace is the output from SS (PORT B, 0) which looks as if it is driving a capacitive load.
When I disable the SPI by setting SPCR to 0x00 I get good fast edges to the pulse which suggests to me that it not the hardware doing this. Using the unaltered hardware I can bit-bash the pins and I hve loaded an Arduino sketch and both methods drive the shift register properly.
I have been through the code umpteen times and I can't see what is wrong with it so I will be grateful for any help.
Thank you in advance Roger S
