PCa9557 I2C using

Hello Bob!

Thanks for the question.

First off, is there a reason you're looking at the PCA9557 over our more recent IO expanders? Our newest IO expanders feature 'TCA' prefixes instead of PCA. They add lower-voltage support and the power on errata bug fix.

Worth noting on the PCA9557, the P0 port is open-drain only. This means P0 can only pull-down or release the pin.

If you would like a comparable device with all 8 ports offering push-pull, then the TCA9534 would be the best cross for this part.

Now, regardless of which device you choose, the following answers your questions will be the same:

1) Can each chip be configured one time at startup for which register will be access from normal SMBus transactions (2 byte) from that point on?

- No. These IO expanders will require the new standard I2C address, Register Address, Data format for all writes and I2C address, REgister Address, Repeated start with address, Read data, for reading.

2) It looks like each output requires 3 bytes instead of the normal two. Can output chips be configured for output and then just use a 2 byte message?

- No. This 2 byte standard became more-or-less superseded with this newer 3 byte style of writing. The reason for this is, the newer IO expanders offer more features than the old style, giving you the ability to independently configure each port as an input or output. Some even allow you to turn on and off pull-up resistors by writing to a register. These additional features require that you explicitly tell the device which register you're writing to.

3)For reading data, can I configure it to use the input data register one time at start up, and then just use normal 2 byte ttransaction?

- Yes and no. This is because of the same reason above. Per the newer I2C format for more advanced devices, you must first tell the device which register you want to read from, then you perform a read address.

However, you can partially do what you want to do if you are going to be ONLY reading from a device. The device will remember the last register accessed, and will always go to that register until told otherwise. So you can write the INPUT register address to the device (the 4 byte read format you saw in the datasheet), then every read you perform to that device afterwards will read from the input register. Note that this will work on IO expanders which do NOT have the auto-incrementing feature. The auto-increment feature will automatically advance the register by 1 so that you can continuously read bytes of data, and the register will automatically increase. Not all devices feature this.

4) Is there any way to make it conform to the normal I2C interface by a one time setup before use

- Again,  no. This is because these IO expanders are more complicated than the traditional IO expanders, which require that multiple registers be used to specify device operation. This is considered the standard I2C format for more advanced slave devices, as opposed to simple sensors or single-function IO expanders.