; Functionality:
;
; This code implements the basic functions for an I2C slave device
; using the SSP module. All I2C functions are handled in an ISR.
; Bytes written to the slave are stored in a buffer. After a number
; of bytes have been written, the master device can then read the
; bytes back from the buffer.
;
; Variables and Constants used in the program:
;
; The start address for the receive buffer is stored in the variable
; 'RXBuffer'. The length of the buffer is denoted by the constant
; value 'RX_BUF_LEN'. The current buffer index is stored in the 
; variable 'Index'.
;
;--------------------------------------------------------------------
; 
; The following files should be included in the MPLAB project:
;
; an734.asm-- Main source code file
;
; 16f872.lkr-- Linker script file
; (change this file for the device
; you are using)
;---------------------------------------------------------------------
#include <p16f872.inc> ; Change to device that you are using.
;---------------------------------------------------------------------
;Constant Definitions
;---------------------------------------------------------------------
#define NODE_ADDR 0x02 ; I2C address of this node
; Change this value to address that
; you wish to use.
;---------------------------------------------------------------------
; Buffer Length Definition
;---------------------------------------------------------------------

#define RX_BUF_LEN 32 ; Length of receive buffer

;---------------------------------------------------------------------
; Variable declarations
;---------------------------------------------------------------------
udata

WREGsave res 1
STATUSsave res 1
FSRsave res 1
PCLATHsave res 1

Index res 1 ; Index to receive buffer
Temp res 1 ;
RXBuffer res RX_BUF_LEN ; Holds rec'd bytes from master
; device.

;---------------------------------------------------------------------
; Vectors
;---------------------------------------------------------------------

STARTUP code
nop 
goto Startup ; 
nop ; 0x0002
nop ; 0x0003
goto ISR ; 0x0004

PROG code

;---------------------------------------------------------------------
; Macros
;---------------------------------------------------------------------

memset macro Buf_addr,Value,Length

movlw Length ; This macro loads a range of data memory
movwf Temp ; with a specified value. The starting
movlw Buf_addr ; address and number of bytes are also 
movwf FSR ; specified.
SetNext movlw Value
movwf INDF
incf FSR,F
decfsz Temp,F
goto SetNext
endm


LFSR macro Address,Offset ; This macro loads the correct value
movlw Address ; into the FSR given an initial data 
movwf FSR ; memory address and offset value.
movf Offset,W
addwf FSR,F
endm

;---------------------------------------------------------------------
; Main Code
;---------------------------------------------------------------------

Startup
bcf STATUS,RP1
bsf STATUS,RP0
call Setup

Main clrwdt ; Clear the WDT 
goto Main ; Loop forever. 


;---------------------------------------------------------------------
; Interrupt Code
;---------------------------------------------------------------------

ISR
movwf WREGsave ; Save WREG
movf STATUS,W ; Get STATUS register
banksel STATUSsave ; Switch banks, if needed.
movwf STATUSsave ; Save the STATUS register
movf PCLATH,W ;
movwf PCLATHsave ; Save PCLATH
movf FSR,W ;
movwf FSRsave ; Save FSR

banksel PIR1
btfss PIR1,SSPIF ; Is this a SSP interrupt? 
goto $ ; No, just trap here.
bcf PIR1,SSPIF
call SSP_Handler ; Yes, service SSP interrupt.

banksel FSRsave
movf FSRsave,W ;
movwf FSR ; Restore FSR
movf PCLATHsave,W ; 
movwf PCLATH ; Restore PCLATH
movf STATUSsave,W ;
movwf STATUS ; Restore STATUS
swapf WREGsave,F ;
swapf WREGsave,W ; Restore WREG

retfie ; Return from interrupt.

;---------------------------------------------------------------------
Setup
;
; Initializes program variables and peripheral registers.
;---------------------------------------------------------------------

banksel PCON
bsf PCON,NOT_POR
bsf PCON,NOT_BOR

banksel Index ; Clear various program variables
clrf Index

clrf PORTB
clrf PIR1
banksel TRISB
clrf TRISB

movlw 0x36 ; Setup SSP module for 7-bit 
banksel SSPCON
movwf SSPCON ; address, slave mode
movlw NODE_ADDR
banksel SSPADD
movwf SSPADD
clrf SSPSTAT


banksel PIE1 ; Enable interrupts
bsf PIE1,SSPIE
bsf INTCON,PEIE ; Enable all peripheral interrupts
bsf INTCON,GIE ; Enable global interrupts

bcf STATUS,RP0
return

;---------------------------------------------------------------------
SSP_Handler
;---------------------------------------------------------------------
; The I2C code below checks for 5 states:
;---------------------------------------------------------------------
; State 1: I2C write operation, last byte was an address byte.
;
; SSPSTAT bits: S = 1, D_A = 0, R_W = 0, BF = 1
;
; State 2: I2C write operation, last byte was a data byte.
;
; SSPSTAT bits: S = 1, D_A = 1, R_W = 0, BF = 1
;
; State 3: I2C read operation, last byte was an address byte.
;
; SSPSTAT bits: S = 1, D_A = 0, R_W = 1, BF = 0
;
; State 4: I2C read operation, last byte was a data byte.
;
; SSPSTAT bits: S = 1, D_A = 1, R_W = 1, BF = 0
;
; State 5: Slave I2C logic reset by NACK from master.
; 
; SSPSTAT bits: S = 1, D_A = 1, R_W = 0, BF = 0
;
; For convenience, WriteI2C and ReadI2C functions have been used.
;----------------------------------------------------------------------

banksel SSPSTAT
movf SSPSTAT,W ; Get the value of SSPSTAT
andlw b'00101101' ; Mask out unimportant bits in SSPSTAT.
banksel Temp ; Put masked value in Temp
movwf Temp ; for comparision checking.

State1: ; Write operation, last byte was an
movlw b'00001001' ; address, buffer is full.
xorwf Temp,W ; 
btfss STATUS,Z ; Are we in State1?
goto State2 ; No, check for next state.....

memset RXBuffer,0,RX_BUF_LEN ; Clear the receive buffer.
clrf Index ; Clear the buffer index.
call ReadI2C ; Do a dummy read of the SSPBUF.
return

State2: ; Write operation, last byte was data,
movlw b'00101001' ; buffer is full.
xorwf Temp,W
btfss STATUS,Z ; Are we in State2?
goto State3 ; No, check for next state.....

LFSR RXBuffer,Index ; Point to the buffer.
call ReadI2C ; Get the byte from the SSP.
movwf INDF ; Put it in the buffer.
incf Index,F ; Increment the buffer pointer.
movf Index,W ; Get the current buffer index.
sublw RX_BUF_LEN ; Subtract the buffer length.
btfsc STATUS,Z ; Has the index exceeded the buffer length?
clrf Index ; Yes, clear the buffer index.
return

State3: ; Read operation, last byte was an
movlw b'00001100' ; address, buffer is empty.
xorwf Temp,W
btfss STATUS,Z ; Are we in State3?
goto State4 ; No, check for next state.....

clrf Index ; Clear the buffer index.
LFSR RXBuffer,Index ; Point to the buffer
movf INDF,W ; Get the byte from buffer.
call WriteI2C ; Write the byte to SSPBUF
incf Index,F ; Increment the buffer index.
return

State4: ; Read operation, last byte was data,
movlw b'00101100' ; buffer is empty.
xorwf Temp,W
btfss STATUS,Z ; Are we in State4?
goto State5 ; No, check for next state....

movf Index,W ; Get the current buffer index.
sublw RX_BUF_LEN ; Subtract the buffer length.
btfsc STATUS,Z ; Has the index exceeded the buffer length?
clrf Index ; Yes, clear the buffer index.
LFSR RXBuffer,Index ; Point to the buffer
movf INDF,W ; Get the byte
call WriteI2C ; Write to SSPBUF
incf Index,F ; Increment the buffer index.
return

State5:
movlw b'00101000' ; A NACK was received when transmitting
xorwf Temp,W ; data back from the master. Slave logic
btfss STATUS,Z ; is reset in this case. R_W = 0, D_A = 1
goto I2CErr ; and BF = 0
return ; If we aren抰 in State5, then something is 
; wrong.

I2CErr nop
banksel PORTB ; Something went wrong! Set LED
bsf PORTB,7 ; and loop forever. WDT will reset
goto $ ; device, if enabled.
return

;---------------------------------------------------------------------
; WriteI2C
;---------------------------------------------------------------------

WriteI2C
banksel SSPSTAT
btfsc SSPSTAT,BF ; Is the buffer full?
goto WriteI2C ; Yes, keep waiting.
banksel SSPCON ; No, continue.
DoI2CWrite
bcf SSPCON,WCOL ; Clear the WCOL flag.
movwf SSPBUF ; Write the byte in WREG
btfsc SSPCON,WCOL ; Was there a write collision?
goto DoI2CWrite
bsf SSPCON,CKP ; Release the clock.
return

;---------------------------------------------------------------------
ReadI2C
;---------------------------------------------------------------------

banksel SSPBUF
movf SSPBUF,W ; Get the byte and put in WREG
return

end ; End of file