A simple I2C protocol for the C8051F410

by sficarro · Published May 19, 2020 · Updated May 19, 2020

For my electronics class, my project required communicating with an I2C peripheral (MCP4725 DAC, adafruit breakout board).

I got stuck trying to use the ‘410 SMBus; example code from the application note is very complex and I could not get this to work. I was able to communicate with the DAC using an Arduino. After observing the Arduino’s I2C implementation using the Saleae logic analyzer, and reading a great tutorial from TI:

http://www.ti.com/lit/an/slva704/slva704.pdf?&ts=1589762906285

I was able to write a very simple I2C protocol for the 8051:

	; simple I2C protocol for C8051F410 used LAoE bitflip.a51 as a starting point
	; https://learningtheartofelectronics.com/program-listings/silabs-code/lab-c1/

	$NOSYMBOLS ; keeps listing short..
	$INCLUDE (C:\MICRO\8051\RAISON\INC\c8051f410.inc)
	SCL EQU P0.0 ; clock
	SDA EQU P0.1 ; data

	BLUE_LED EQU P0.4 ; an LED is always nice


	ORG 0 ; tells assembler the address at which to place this code

	SJMP STARTUP ; here code begins--with just a jump to start of
	; real program. ALL our programs will start thus
	ORG 80h ; ...and here the program starts

	STARTUP:

	ACALL USUAL_SETUP
	ACALL IO_SETUP

	SETB BLUE_LED
	SETB SCL ; initialize with SCL, SDA high
	SETB SDA

	MAIN: CPL BLUE_LED ; flips LED

	ACALL SET_VOLTAGE_TO_5V ; Comment/uncomment for testing
	//ACALL SET_VOLTAGE_TO_1V
	//ACALL SET_VOLTAGE_TO_0V

	SJMP MAIN

	; These small set_voltage functions demonstrate a how to send a complete I2C frame
	; Start
	; Load accumulator with value, then send byte
	; repeat if needed
	; Stop

	SET_VOLTAGE_TO_5V: ; as advertised, sets the DAC to max 5V
	ACALL I2C_START
	MOV A, #0C4h
	ACALL I2C_SEND_BYTE
	MOV A, #040h
	ACALL I2C_SEND_BYTE
	MOV A, #0FAh
	ACALL I2C_SEND_BYTE
	MOV A, #00h
	ACALL I2C_SEND_BYTE
	LCALL I2C_STOP
	LCALL DELAY
	RET

	SET_VOLTAGE_TO_0V:
	ACALL I2C_START
	MOV A, #0C4h
	ACALL I2C_SEND_BYTE
	MOV A, #040h
	ACALL I2C_SEND_BYTE
	MOV A, #00h
	ACALL I2C_SEND_BYTE
	MOV A, #00h
	ACALL I2C_SEND_BYTE
	LCALL I2C_STOP
	LCALL DELAY
	RET

	SET_VOLTAGE_TO_1V:
	ACALL I2C_START
	MOV A, #0C4h
	ACALL I2C_SEND_BYTE
	MOV A, #040h
	ACALL I2C_SEND_BYTE
	MOV A, #32h
	ACALL I2C_SEND_BYTE
	MOV A, #00h
	ACALL I2C_SEND_BYTE
	LCALL I2C_STOP
	LCALL DELAY
	RET

	I2C_START: ; start condition
	CLR SDA
	CLR SCL
	RET

	I2C_STOP: ; stop condition
	CLR SDA
	CLR SCL
	SETB SCL
	SETB SDA
	RET

	;Send Byte assumess accumulator loaded with byte you want to send.
	;Basically, just reads bit 7 and pulses SDA appropriately, along with a clock pulse.
	;Then, all bits are shifted and the process is repeated until all 8 bits are sent.
	;After the 8 bits, an acknowledgement bit is sent.
	I2C_SEND_BYTE:
	MOV R0, #08
	I2C_SEND_LOOP:
	JB ACC.7, GO_HIGH
	GO_LOW:
	ACALL I2C_LOW
	SJMP CONTINUE
	GO_HIGH:
	ACALL I2C_HIGH
	SJMP CONTINUE
	CONTINUE:
	RL A
	DJNZ R0, I2C_SEND_LOOP
	I2C_SEND_BYTE_DONE:
	ACALL I2C_HIGH //ACK
	RET

	I2C_HIGH:
	SETB SDA ; data high
	SETB SCL ; Clock hi edge
	SETB SDA ;stay high
	CLR SCL ; clear clock
	RET

	I2C_LOW:
	CLR SDA ; data low
	SETB SCL ; clock edge hi
	CLR SDA ; stay low
	CLR SCL ; clear clock
	RET


	DELAY:
	MOV R0, #20h
	WAIT:
	DJNZ R0, WAIT
	RET

	USUAL_SETUP: ; Disable the WDT.
	anl PCA0MD, #NOT(040h) ; Clear Watchdog Enable bit

	; Enable the Port I/O Crossbar

	mov XBR1, #40h ; Enable Crossbar

	ret
	IO_SETUP: orl P0MDOUT, #01h ; enable P0.0 as push-pull output
	RET

	END