PCF8574A IO Expander with nRF52-DK

RE: PCF8574A IO Expander with nRF52-DK

Mark — Thu, 19 May 2016 18:14:58 GMT

I couldn't get my code to work. It compiles and loaded on the board, but doesn't do anything.

First thing I wanted to double check, can I power both the PCF8574A IO Expander and HD44780 LCD through VSHLD(3.3V) pin of the nRF52-DK board? If I have to run HD44780 LCD from a 5V supply, then can I use something like BSS138 MOSFETs to level shift my I2C line from 3.3V to 5V?

Also, do I need to use 10K pull up resistors on the I2C line?

RE: PCF8574A IO Expander with nRF52-DK

Mark — Wed, 18 May 2016 18:01:15 GMT

Yeah, I've never worked with ARM core processors before nor Nordic SDKs; so, I'm trying to get my code working in blocking mode first, and if successful, then I'll try to use twi_handler later. I'll most likely have more questions about how to use a handler later.

RE: PCF8574A IO Expander with nRF52-DK

RK — Wed, 18 May 2016 03:16:42 GMT

oh one more comment - don't send the data on the stack like that- if you're using blocking mode you'll probably get away with it, but using a static buffer is a much better idea.

RE: PCF8574A IO Expander with nRF52-DK

RK — Wed, 18 May 2016 00:16:39 GMT

Yep - boringly simple isn't it, especially when you have only 1 byte of data to send and receive.

RE: PCF8574A IO Expander with nRF52-DK

Mark — Tue, 17 May 2016 18:32:18 GMT

Oh, ok, thanks RK. The reason I want to use an IO Expander is that I will be using the nRF52-DK in a big project and multiple I2C devices, so I want to make it neat.

I know my device address is 0x38, but as Alex pointed out I thought register address is a necessary parameter to use in order to use TWI function.

Now, when I looked up nrf_drv_twi_tx() function, the parameter that you send data is declared as "uint8_t const *". So can I use the TWI function in the following way?

static ret_code_t expanderWrite(uint8_t buffer)
{

return nrf_drv_twi_tx(&m_twi_hd44780, hd44780_ADDR, &buffer, sizeof(buffer), false);

}

passing data as reference &buffer?

RE: PCF8574A IO Expander with nRF52-DK

Alex — Tue, 17 May 2016 10:46:31 GMT

That i forgot. Yes.

RE: PCF8574A IO Expander with nRF52-DK

RK — Tue, 17 May 2016 10:18:06 GMT

well that's not how that function works, it takes the I2C peripheral address plus a buffer of data and a length, that's it, and it writes the data to the device at that address. There is no concept in that function of a register address on the other side, it's just raw data.

so nrf_drv_twi_tx() will work just as it is without rewriting it, and if you actually did set data length to zero then it wouldn't do anything because it wouldn't write any data. He wants data == the 8 bits of data he wants to send out and data length == 1

RE: PCF8574A IO Expander with nRF52-DK

Alex — Tue, 17 May 2016 10:13:12 GMT

Mark talking about address of the register inside of the expander. PCF8574 doesn't have it but standard I2C function designed to work with memory type of device where you have to send internal address then data. This is how I understood.

RE: PCF8574A IO Expander with nRF52-DK

RK — Tue, 17 May 2016 10:07:05 GMT

The PCF8574A has an address, it has to have an address, it's an I2C device, I2C requires things to have an address. The address is whatever you set up on the A0-A2 pins.

So you give the command the address of the expander and write a byte of data.

I do have to ask however, since you have an nRF52 development kit which has masses of output lines just begging to be used, why do you need a port expander? Port expanders are for arduinos which come with about 1 1/2 pins available.

RE: PCF8574A IO Expander with nRF52-DK

Alex — Tue, 17 May 2016 08:35:21 GMT

Either change nrf_drv_twi_tx() or use address as data and length as zero when writing.