Introduction.
This discussion shows how to interface a Philips PCF8574 with a Motorola 68HC11 processor. Two examples are provided. One flashes an LED on P0 of the 8574 when a switch on P7 is depressed. The second turns a stepper on P0 - P3 in a direction determined by a switch on P7 and at a speed determined by three switches on P4 - P6.
Note that the low level I2C routines; SDA_hi, SDA_lo, SCL_hi, SCL_lo, START, STOP, out_byte, in_byte and nack may be used with any I2C device.
The Philips PCF9574 may be obtianed form Newark Electronics (http://www.newark.com). A data sheet is available at http://www.semiconductors.philips.com. As of this writing, their homepage is poorly organized. I found it by doing a search on "PCF8574".
I2C Protocol Routines.
SCL_lo and SCL_hi are accomplished by setting bit 0 of the DDRC register to a logic one (output) and then setting bit 0 of PORTC to either a logic 0 or 1, respectively. Thus, the two clock states are a low impedance to ground and a low impedance to +5.
In SDA_lo, bit 1 of DDRC is set to a logic one (ouput) and bit 1 of PORTC to a logic zero.
SDA_hi is a bit different. Bit 1 of DDRC is set to a logic zero, thus setting the associated bit on PORTC to an input, resulting in a high impedance.
Thus, the two states associated with SDA are a low impedance to ground (logic 0) and an open circuit (logic 1).
START is implemented by bring SDA for a logic one to logic zero while SCL is high. STOP is similar; SDA is brought high while SCL is high. Note that at all other times, SDA is changed only when SCL is low.
Subroutine out_byte shifts out variable o_byte beginning with the most significant bit. SCL is low. SDA is set to the state associated with the most significant bit of o_byte. SCL is then brought high and then low. Variable o_byte is then shifted left such that bit 6 is now the most significant bit and the process is repeated for all eight bits.
Subroutine nack is implemented by simply bringing SDA high and then bringing SCL high and then low. This allows the 8574 to acknowledge the byte.
In function in_byte, SDA is configured as an input. SCL is brought high and the SDA input is read. SCL is then brought low. The result of the read is located in i_byte.
8574 Routines.
Subroutine out_patt outputs content of variable patt to the 8574. This is accomplished by first exerting the "start" sequence, followed by the address byte with the R/W bit set to logic zero to indicate the operation is a write. After a nack, the data to be written is then sent followed by the "stop" sequence.
START 0100 AAA 0 N DDDD DDDD N STOP
210
where 0100 is the manufacturers defined group address for the 8574. A2, A1 and A0 are the states the user has defined in wiring the A2, A1 and A0 leads. (In these examples, I assummed 000).
Note that N indicates a nack and DDDD DDDD indicates the data to appear on the output of the 8574.
Note that outputs of the 8574 are to be in a logic one state if they are used as inputs. Thus, a variable "dirs" has been defined to specify which bits are inputs. In the LED example, "dirs" has been set to 0x80 indicating that the most significant pin of the 8574 is an input.
Subroutine out_patt then ors the patt with dirs and ouputs the result so as to assure the designated input bits are set to a logic one.
The 8574 is read by exerting the "start" sequence, followed by the address byte with the R/W bit set to logic one to indicate a read. A nack is then sent, followed by an 8-bit read.
START 0100 AAA 1 N RRRR RRRR N STOP 210
the result is returned in variable i_patt. Note that only the bits which have been defined as inputs have meaning.
Program 8574_1.ASM
The input of the 8574 is read using subroutine in_patt. The state of the most significant bit is then tested using the sign flag. If "plus", indicating the most significant bit is a logic zero, the LED is flashed once. Otherwise, the LED is turned off.
In flashing the LED, 8574 output P0 is brought low turning on the LED for a time determined by the delay and then to a logic one turning the LED off. This is then fllowed by a delay.
* Program 8574_1.ASM * * Illustrates interface with PCF8574 8-bit I/O Expander using I2C * protocol. * * Pulses LED on least significant bit of 8574 when switch on most * significant bit is in one state. * * PCF8574 controlled via PORTC, bit 0 - SCL, bit 1 - SDA * * Copyright, Peter H. Anderson, MSU, 23 April 97 * PSCT EQU $C000 DSCT EQU $D000 IDSCT EQU $D300 DDRC EQU $07 PORTB EQU $04 PORTC EQU $03 ORG PSCT LDS #$0040 LDY #$1000 ldaa #%10000000 staa dirs * ms bit is an input ldaa #$ff * set all outputs to an intial value of 1 staa o_patt bsr out_byte top bsr in_patt * get the inputs tst i_patt bpl flash * if most sign input is zero bmi no_flash flash ldaa #$00 * turn on the LED staa o_patt bsr out_patt bsr delay ldaa #$01 * turn it off staa o_patt bsr out_patt bsr delay bra top no_flash ldaa #$01 * turn off the LED staa o_patt bsr out_patt bsr delay bra top delay pshx * delay for about 250 ms ldx #$ffff delay_L1 dex bne delay_L1 pulx rts in_patt bsr START ldaa #$41 * address - read staa o_byte bsr out_byte bsr nack bsr in_byte * fetch the input bsr nack bsr STOP ldaa i_byte staa i_patt * return result in i_patt rts out_patt bsr START ldaa #$40 * address - write staa o_byte bsr out_byte bsr nack ldaa o_patt * pattern to be output oraa dirs * be sure pins designated as inputs are at logic one staa o_byte bsr out_byte bsr nack bsr STOP rts in_byte bsr SDA_hi * be sure SDA is an input ldaa #8 staa n clr i_byte L1_in bsr SCL_hi ldaa PORTC,Y asra * align bit 1 in bit 0 position anda #$01 * bit only asla i_byte * shift i_byte and or the bit oraa i_byte staa i_byte bsr SCL_lo dec n bne L1_in rts out_byte ldaa #8 staa n L1_out tst o_byte bpl out_0 * SDA set high or low based on ms bit bmi out_1 L2_out bsr SCL_hi bsr SCL_lo asl o_byte dec n bne L1_out rts out_0 bsr SDA_lo bra L2_out out_1 bsr SDA_hi bra L2_out ** nack bsr SDA_hi * bring SDA to logic one and clock bsr SCL_hi bsr SCL_lo rts START bsr SCL_lo bsr SDA_hi bsr SCL_hi bsr SDA_lo * SDA 1 to 0 when SCL is high bsr SCL_lo rts STOP bsr SCL_lo bsr SDA_lo bsr SCL_hi bsr SDA_hi * SDA 0 to 1 when SCL is high bsr SCL_lo rts SCL_hi bset DDRC,Y %00000001 * make bit 0 on PORTC an output bset PORTC,Y %00000001 rts SCL_lo bset DDRC,Y %00000001 bclr PORTC,Y %00000001 rts SDA_hi bclr DDRC,Y %00000010 * make bit 1 an input rts SDA_lo bclr PORTC,Y %00000010 bset DDRC,Y %00000010 rts ORG DSCT dirs rmb 1 i_patt rmb 1 o_patt rmb 1 o_byte rmb 1 i_byte rmb 1 n rmb 1
Program 8574_2.ASM
Note that the low four bits are defined as outputs, driving a stepper using a ULN2803 driver of similar. The high four bits are inputs. A switch at P7 determines the direction. Switches at P4, P5 and P6 determine the speed of rotation.
The inputs are read using subroutine in_patt. Bits 4, 5 and 6 are isolated and copied to variable "speed".
The most significant (direction) bit is then tested and program control is transferred to either CW or CCW. In CW, pointer X is walked down through the array of stepping motor patterns and set back to PATTS when it gets beyond PATTS+7. In CCW, pointer X is walked up and set back to PATTS+7 when it is less than PATTS.
The pattern pointed to by X is output using subroutine out_patt. This is followed by a delay.
In the delay function, the content of variable "speed" is multipied by 0xff to determine the intial state of the counter which is then decremented to zero.
* Program 8574_2.ASM
*
* Illustrates how an 8574 might be used to control a stepping motor.
* Four coils of stepper controlled by lowest four bits on 8574.
*
* Switch on most significant bit (P7) of 8574 controls direction of motor.
* Speed is controlled by switches on P6, P5 and P4 of 8574
*
* 8574 is controlled by PORTC; bit 0 - SCL, bit 1 - SDA
*
* coyright Peter H. Anderson, MSU, April 23, '97
*
PSCT EQU $C000
DSCT EQU $D000
IDSCT EQU $D300
DDRC EQU $07
PORTB EQU $04
PORTC EQU $03
ORG PSCT
LDS #$0040
LDY #$1000
ldaa #%11110000
staa dirs * ms nibble is an input
ldaa #ff * intialize all outputs to logic 1
staa o_patt
bsr out_byte
ldx #PATTS * intialize pointer
top
bsr in_patt * fetch switch to determine direction and
speed
ldaa i_patt
asra
asra
asra
asra
anda #$07
staa speed
tst i_patt * direction bit
bpl CW * if direction bit is zero
bmi CCW
CW inx * advance the pointer
cpx #PATTS+8 * if too far, set back to beginning
bne CW_around
ldx #PATTS
CW_around
ldaa 00,x
staa o_patt * get the pattern and output it
jsr out_patt
jsr delay
bra top
CCW dcx * dec the pointer
cpx #PATTS-1 * it too far, set at end
bne CCW_around
ldx #PATTS+7
CCW_around
ldaa 00,x
staa o_patt
jsr out_patt
jsr delay
bra top
***
delay
pshx * delay for period
psha
pshb
ldaa speed * multiply speed by 0xff
ldab $ff
mul
xgdx * result is now in X
delay_L1
dcx
bne delay_L1
pulb
pula
pulx
rts
***
in_patt
jsr START * reads 8574. returns result in i_patt
ldaa #$41
staa o_byte
jbsr out_byte
jsr nack
jsr in_byte
jsr nack
jsr STOP
ldaa i_byte
staa i_patt
rts
out_patt
bsr START
ldaa #$40
staa o_byte
jsr out_byte
jsr nack
ldaa o_patt
oraa dirs
staa o_byte
jsr out_byte
jsr nack
bsr STOP
rts
in_byte
SDA_hi * be sure SDA is an input
ldaa #8
staa n
clr i_byte
L1_in
SCL_hi
ldaa PORTC,Y
asra * align bit 1 in bit 0 position
anda #$01 * bit only
alsa i_byte * shift i_byte and or the bit
oraa i_byte
staa i_byte
SCL_lo
dec n
bne L1_in
rts
out_byte
ldaa #8
staa n
L1_out
tst o_byte
bpl out_0 * SDA set high or low based on ms bit
bmi out_1
L2_out
jsr SCL_hi
jsr SCL_lo
asl o_byte
dec n
bne L1_out
rts
out_0
jsr SDA_lo
bra L2_out
out_1
jsr SDA_hi
bra L2_out
**
nack
jsr SDA_hi * SDA to high impedance and clock
jsr SCL_hi
jsr SCL_lo
rts
START
jsr SCL_lo
jsr SDA_hi
jsr SCL_hi
jsr SDA_lo * SDA 1 to 0 when SCL is high
jsr SCL_lo
rts
STOP
jsr SCL_lo
jsr SDA_lo * SDA 0 to 1 when SCL is high
jsr SCL_hi
jsr SDA_hi * SDA 0 to 1 when SCL is high
jsr SCL_lo
rts
SCL_hi
bset DDRC,Y %00000001 * make bit 0 on PORTC an output
bset PORTC,Y %00000001
rts
SCL_lo
bset DDRC,Y %00000001
bclr PORTC,Y %00000001
rts
SDA_hi
bclr DDRC,Y %00000010 * make bit 1 an input
rts
SDA_lo
bclr PORTC,Y %00000010
bset DDRC,Y %00000010
rts
ORG DSCT
dirs rmb 1
i_patt rmb 1
o_patt rmb 1
o_byte rmb 1
i_byte rmb 1
n rmb 1
speed rmb 1
ORG IDSCT
PATTS FCB $01, $03, $02, $06
FCB $04, $0c, $08, $09