read_random:

	; AT24Cxx Random Read function.
	; Called with programmable address in A, byte address in
	; register pair ADDR_HI:ADDR_LO. Returns data in A.
	; Returns CY set to indicate that the bus is not available
	; or that the addressed device failed to acknowledge.
		push	b
		mov	b, a		; save copy of programmable address

		; Send dummy write command to set internal address.

		call	start
		jc	x47		; abort if bus not available

		rl	a		; programmable address to bits 3:1
		orl	a, #FADDR	; add fixed address
		clr	acc.0		; specify write operation
		call	shout		; send device address
		jc	x46		; abort if no acknowledge


		mov	a, addr_hi	; send high byte of address
		call	shout		;
		jc	x46		; abort if no acknowledge



		mov	a, addr_lo	; send low byte of address
		call	shout		;
		jc	x46		; abort if no acknowledge

		; Call Current Address Read function.





		mov	a, b		; get programmable address
		call	read_current
		jmp	x47		; exit
	x46:
		call	stop
	x47:
		pop	b

                lcall DL512
	
		ret





read_current:

	; AT24Cxx Current Address Read function.
	; Called with programmable address in A. Returns data in A.
	; Returns CY set to indicate that the bus is not available
	; or that the addressed device failed to acknowledge.

		call	start
		jc	x45		; abort if bus not available




		rl	a		; programmable address to bits 3:1
		orl	a, #FADDR	; add fixed address
		setb	acc.0		; specify read operation
		call	shout		; send device address
		jc	x44		; abort if no acknowledge




		call	shin		; receive data byte
		call	NAK		; do not acknowledge byte
		clr	c		; clear error flag
	x44:
		call	stop
	x45:
		ret







write_byte:

	; AT24Cxx Byte Write function.
	; Called with programmable address in A, byte address in
	; register pair ADDR_HI:ADDR_LO, data in register XDATA.
	; Does not wait for write cycle to complete.
	; Returns CY set to indicate that the bus is not available
	; or that the addressed device failed to acknowledge.
	; Destroys A.
                
		call	start
		jc	x49		; abort if bus not available

		rl	a		; programmable address to bits 3:1
		orl	a, #FADDR	; add fixed address
		clr	acc.0		; specify write operation
		call	shout		; send device address
		jc	x48		; abort if no acknowledge

		mov	a, addr_hi	; send high byte of address	       

		call	shout		;
		jc	x48		; abort if no acknowledge


		mov	a, addr_lo	; send low byte of address

     		call	shout		;
		jc	x48		; abort if no acknowledge

		mov	a, zdata	; get data
		call	shout		; send data
		jc	x48		; abort if no acknowledge

		clr	c		; clear error flag
	x48:
		call	stop
	x49:    acall   DL512
 
		ret








start:

	; Send START, defined as high-to-low SDA with SCL high.
	; Return with SCL, SDA low.
	; Returns CY set if bus is not available.

		setb	SDA
		setb	SCL

		; Verify bus available.

		jnb	SDA, x40	; jump if not high
		jnb	SCL, x40	; jump if not high

		nop			; enforce setup delay and cycle delay
		clr	SDA
		nop			; enforce hold delay
		nop			;
		nop			;
		nop			;
		nop			;
		clr	SCL

		clr	c		; clear error flag
		jmp	x41
	x40:
		setb	c		; set error flag
	x41:
		ret




stop:

	; Send STOP, defined as low-to-high SDA with SCL high.
	; SCL expected low on entry. Return with SCL, SDA high.

		clr	SDA
		nop			; enforce SCL low and data setup
		nop
		setb	SCL
		nop			; enforce setup delay
		nop			;
		nop			;
		nop			;
		nop			;
		setb	SDA
		ret

shout:

	; Shift out a byte to the AT24Cxx, most significant bit first.
	; SCL, SDA expected low on entry. Return with SCL low.
	; Called with data to send in A.
	; Returns CY set to indicate failure by slave to acknowledge.
	; Destroys A.

		push	b
		mov	b, #8		; bit counter
	x42:
		rlc	a		; move bit into CY
		mov	SDA, c		; output bit
		nop			; enforce SCL low and data setup
		setb	SCL		; raise clock
		nop			; enforce SCL high
		nop			;
		nop			;
		nop			;
		clr	SCL		; drop clock
		djnz	b, x42		; next bit

		setb	SDA		; release SDA for ACK
		nop			; enforce SCL low and tAA
		nop			;
		setb	SCL		; raise ACK clock
		nop			; enforce SCL high
		nop			;
		nop			;
		nop			;
		mov	c, SDA		; get ACK bit
		clr	SCL		; drop ACK clock

		pop	b
		ret


shin:

	; Shift in a byte from the AT24Cxx, most significant bit first.
	; SCL expected low on entry. Return with SCL low.
	; Returns received data byte in A.

		setb	SDA		; make SDA an input

		push	b
		mov	b, #8		; bit count
	x43:
		nop			; enforce SCL low and data setup
		nop			;
		nop			;
		setb	SCL		; raise clock
		nop			; enforce SCL high
		nop			;
		mov	c, SDA		; input bit
		rlc	a		; move bit into byte
		clr	SCL		; drop clock
		djnz	b, x43		; next bit

		pop	b
		ret





ACK:

	; Clock out an acknowledge bit (low).
	; SCL expected low on entry. Return with SCL, SDA low.

		clr	SDA		; ACK bit
		nop			; enforce SCL low and data setup
		nop			;
		setb	SCL		; raise clock
		nop			; enforce SCL high
		nop			;
		nop			;
		nop			;
		clr	SCL		; drop clock
		ret


NAK:
	; Clock out a negative acknowledge bit (high).
	; SCL expected low on entry. Return with SCL low, SDA high.

		setb	SDA		; NAK bit
		nop			; enforce SCL low and data setup
		nop			;
		setb	SCL		; raise clock
		nop			; enforce SCL high
		nop			;
		nop			;
		nop			;
		clr	SCL		; drop clock
		ret