PCF8574 I/O Expander

13 March 2021
The Texas Instruments PCF8574 is pretty much the first I/O Expander chip and first I²C device I ever used and it is a contender for the chip I have used the most when creating circuits. Being zero-configuration I have often used it for test purposes and finally decided to make a permanent test circuit based around the chip; this demonstration circuit is shown below. Since I had not used the chip for some time and did not have the data-sheets at hand I decided to write a mini-article with all the relevant information so that I would not need to go digging for it again.

PCF8574 demo circuit

Chip pinning

The chip pinning for the PCF8574 is summetised in the table below, where Ax are address pins and Px are input/output pins. On chip reset all of the latter pins are set to high-impedance and hence input rather than output.

V_cc	SDA	SCL	Int	P7	P6	P5	P4
16	15	14	13	12	11	10	9
1	2	3	4	5	6	7	8
A0	A1	A2	P0	P1	P2	P3	Gnd

Upon an I²C write pins that have a high written become high-impedance whereas those with a low written will sink current; upon a subsequent read pins sinking current will always read as low whereas the hi-impedence pins will read high or low dependent on external driving. The interrupt pin like the input/output pins is open-drain, and is active-low so it will conduct when the input value that would be read is different to the previous I²C transaction; this means an interrupt will be deasserted if the input reverts to the previous value. The I²C address is set by the A0 thru A2 pins as per the table below.

Address pins			I²C
`A0`	`A1`	`A2`	`Read`	`Write`
`0`	`0`	`0`	`0x41`	`0x40`
`1`	`0`	`0`	`0x43`	`0x42`
`0`	`1`	`0`	`0x45`	`0x44`
`1`	`1`	`0`	`0x47`	`0x46`
`0`	`0`	`1`	`0x49`	`0x48`
`1`	`0`	`1`	`0x4b`	`0x4a`
`0`	`1`	`1`	`0x4d`	`0x4c`
`1`	`1`	`1`	`0x4f`	`0x4e`

Test circuit schematic

The essential parts of the test circuit is summerised in the simplified schmatic shown below — the idea is that pins used for output should have the associated tri-state DIP switch set to open-circuit, but the wiring is such that there are no ill-effects such as a low-resistance path between power rails if this is not done.

Simplified switch schematic

Remarks

I actually built this circuit back in February when I was in the UK with all my equipment, but decided to hold back on finishing off this write-up as I had other things that needed my immediate attention. As with the LCD timer encosure this circuit was expected to be the last electronics I will do for some considerable time.