;**** A P P L I C A T I O N   N O T E   A V R 3 2 0 *****************
;*
;* Title		: Software SPI Master
;* Version		: 1.0
;* Last updated		: 98.04.21
;* Target		: AT90S1200
;* Easily modified for	: Any AVR microcontroller
;*
;* Support E-mail	:avr@atmel.com
;*
;* DESCRIPTION
;*	This is a collection of 8/16-bit word, Mode 0, Master SPI routines.
;*	It simultaneously transmits and receives SPI data in 8- or 16-bit
;*	word format.  Data is sent and received MSB-first.  One pair of
;*	registers is used both to send and to receive; i.e., when one bit
;*	is shifted out (transmitted), the vacated bit position is used to
;*	store the new received bit.  These routines are low-level
;*	interface routines, and do not inherently contain a command
;*	structure; that is dictated by the connected SPI peripheral(s).
;*
;*	Due to having separate Enable/Disable and Read/Write-Word
;*	routines, larger blocks of data can be sent simply by calling
;*	the RW_SPI routine multiple times before disabling /SS.
;*
;* MAJOR ROUTINES:
;*	init_spi: initializes the port lines used for SPI.
;*			No calling requirements, returns nothing.
;*	ena_spi:  forces SCK low, and activates /SS signal.
;*			No calling requirements, returns nothing.
;*	disa_spi: brings /SS signal hi (inactive).
;*			No calling requirements, returns nothing.
;*	rw_spi:   sends/receives a an 8-bit or 16-bit data word.
;*			Must set up data to be sent in (spi_hi,spi_lo)
;*			prior to calling; it returns received data in
;*			the same register pair (if 8-bit, uses only
;*			the spi_lo register).
;*
;* VARIABLES:
;*	The spi_hi and spi_lo variables are the high and low data bytes.
;*	They can be located anywhere in the register file.
;*	
;*	The temp variable holds the bit count, and is also used in timing
;*      the high/low minimum pulse width.  This must be located in an
;*	upper register due to the use of an IMMEDIATE-mode instruction.
;*
;* HISTORY
;*	V1.0	98.04.21 (rgf)  Created
;*		
;***************************************************************************



;**** includes ****

.include "1200def.inc"		;you can change this to any device




;***************************************************************************
;*
;* CONSTANTS
;*
;***************************************************************************

;**** Revision Codes ****

.equ	SW_MAJOR = 1		; Major SW revision number
.equ	SW_MINOR = 0		; Minor SW revision number
.equ	HW_MAJOR = 0		; Major HW revision number
.equ	HW_MINOR = 0		; Minor HW revision number



;***************************************************************************
;*
;* PORT DEFINITIONS
;*
;***************************************************************************

.equ	sck	= 0		;PB0 pin
.equ	nss	= 1		;PB1 pin
.equ	mosi	= 2		;PB2 pin
.equ	miso	= 3		;PB3 pin



;***************************************************************************
;*
;* REGISTER DEFINITIONS
;*
;***************************************************************************

.def	spi_lo	=r0	;change as needed
.def	spi_hi	=r1	;  "
.def	temp	=r16	;misc usage, must be in upper regs for IMMED mode




;***************************************************************************
;*
;* MACROS
;*	Program Macros
;*
;* DESCRIPTION
;*	Change the following macros if a port other than PORTB is used.
;*
;***************************************************************************

.macro	ss_active
	cbi	portb,nss
.endm


.macro	ss_inactive
	sbi	portb,nss
.endm


.macro	sck_hi
	sbi	portb,sck
.endm


.macro	sck_lo
	cbi	portb,sck
.endm


.macro	mosi_hi
	sbi	portb,mosi
.endm


.macro	mosi_lo
	cbi	portb,mosi
.endm


.macro	addi
	subi	@0, -@1		;subtract the negative of an immediate value
.endm

.macro	set_delay		;set up the time delay amount, from 1 to 7
	subi	@0, (@1 << 5)	;NOTE: THIS shift affects INC macro (below)!
.endm

.macro	inc_delay		;bump the delay counter
	subi	@0, -(1 << 5)	;shift value here must be same as above!
.endm




;***************************************************************************
;*
;* SAMPLE APPLICATION, READY TO RUN ON AN AT90S1200
;*
;***************************************************************************

.cseg
.org	0
Rvect:	rjmp	Reset




;***************************************************************************
;*
;* FUNCTION
;*	init_spi
;*
;* DESCRIPTION
;*	Initialize our port pins for use as SPI master.
;*
;* CODE SIZE:
;*	8 words
;*
;***************************************************************************

init_spi:
	ss_inactive		;set latch bit hi (inactive)
	sbi	ddrb,nss	;make it an output
	;
	sck_lo			;set clk line lo
	sbi	ddrb,sck	;make it an output
	;
	mosi_lo			;set data-out lo
	sbi	ddrb,mosi	;make it an output
	;
	cbi	ddrb,miso	;not really required, it powers up clr'd!
	ret



;***************************************************************************
;*
;* FUNCTION
;*	ena_spi
;*
;* DESCRIPTION
;*	Init data & clock lines, then assert /SS.  Note that if more than
;*	one slave is used, copies of this could be made that would each
;*	reference a different /SS port pin (use SS_ACTIVE0, SS_ACTIVE1, ...)
;*
;* CODE SIZE:
;*	4 words
;*
;***************************************************************************

ena_spi:
	sck_lo			;(should already be there...)
	mosi_lo
	ss_active
	ret


;***************************************************************************
;*
;* FUNCTION
;*	disa_spi
;*
;* DESCRIPTION
;*	De-assert /SS.  Since this routine is so short, it might be better
;*	to use the SS_INACTIVE statement directly in higher level code.
;*	Again, if multiple slaves exist, additional copies of this could
;*	be created; or ONE routine that disabled ALL /ss signals could be
;*	used instead to make the code less error-prone due to calling the
;*	wrong Disable routine.
;*
;* CODE SIZE:
;*	2 words
;*
;***************************************************************************

disa_spi:
	ss_inactive
	ret






;***************************************************************************
;*
;* FUNCTION
;*	rw_spi
;*
;* DESCRIPTION
;*	Write a word out on SPI while simultaneously reading in a word.
;*	Data is sent MSB-first, and info read from SPI goes into
;*	the same buffer that the write data is going out from.
;*	Make sure data, clock and /SS are init'd before coming here.
;*	SCK high time is ((delay * 3) + 1) AVR clock cycles.
;*
;*	If 8-bit use is needed, change  LDI TEMP,16  to ,8  and also
;*	eliminate the ROL SPI_HI statement.
;*
;* CODE SIZE:
;*	21 words
;* NUMBER OF CYCLES:
;*	Overhead = 8, loop = 16 * (16 + (2* (delay_value*3)))
;	(With call + return + delay=4, it is about 648 cycles.)
;*
;***************************************************************************

rw_spi:	
	ldi	temp,16		;init loop counter to 16 bits
       ;ldi	temp,8		;use THIS line instead if 8-bit desired
	;
spi_loop:
	lsl	spi_lo		;move 0 into D0, all other bits UP one slot,
	rol	spi_hi		; and C ends up being first bit to be sent.
 ;If 8-bit desired, also comment out the preceding ROL SPI_HI statement
	;
	brcc	lo_mosi
	mosi_hi
	rjmp	mosi_done	;this branch creates setup time on MOSI
lo_mosi:
	mosi_lo
	nop			;also create setup time on MOSI
mosi_done:
	;
	sck_hi
	;
 ;must now time the hi pulse - not much else we can do here but waste time
	;
	set_delay temp,4	;(4 * 3) cycle delay; range is from 1 to 7!
time_hi:
	inc_delay temp		;inc upper nibble until it rolls over; then,
	brcs	time_hi		; C gets CLEARED, & temp has original value
	;
	sck_lo			;drop clock line low
	;
 ;must now delay before reading in SPI data on MISO
	;
	set_delay temp,4
time_lo:
	inc_delay temp
	brcs	time_lo
	;
	sbic	pinb,miso	;after delay, read in SPI bit & put into D0
	inc	spi_lo		;we FORCED D0=0, so use INC to set D0.
	;
	dec	temp
	brne	spi_loop
	ret

;************************  End of SPI routines ****************************

;**** Application example ****

Reset:	rcall	init_spi
	ser	temp		;load w/ FF
	out	DDRD,temp
	rjmp	Main


Main:	ldi	R22,0xA3	;misc data
	mov	spi_lo,R22	;set up information to be sent
	mov	spi_hi,R22	;COMMENT THIS OUT IF 8-BIT MODE
	rcall	ena_spi		;activate /SS
	rcall	rw_spi		;send/receive 16 bits (or 8 bits)
	rcall	disa_spi	;deactivate /SS
	rcall	use_spi_rcv	;go use whatever we received
	rjmp	Main


Use_spi_rcv:			;just copy rcv'd data to Port D pins
	out	PortD,R22
	ret

;**** End of File ****