RE: nRF52832 TWI implementation with 16-bit register addresses

Jackson Wilson — Tue, 26 Apr 2022 13:48:22 GMT

I had already reviewed all those links before I submitted my question. They were not specific enough.

I do mean "register address", not "device address".

My question is very specifically about the nrf_drv_twi_tx() function. An I2C read is a two-part process: (1) I2C write register address, (2) I2C read from that address. So a nrf_drv_twi_tx() is followed by nrf_drv_twi_rx().

The answer to my question is to break the register address into an array of uint8_t type and write the bytes sequentially. So a register address of 0x1234 can be written as 2 bytes:

uint8_t reg[] = {0x12, 0x34};  //decompose 16-bit address, MSB first
err_code = nrf_drv_twi_tx(&m_twi, DVC_I2C_ADDR, reg, 2, false);

This simple answer hopefully will help others who ask the same question about 16-bit and 32-bit register addressing using the nrf_drv_twi_tx() question.

RE: nRF52832 TWI implementation with 16-bit register addresses

JaTi — Tue, 26 Apr 2022 06:14:41 GMT

Here a user describes accessing a device which uses a 32 bit address, maybe the proposed configuration is interesting for you.
In this question another user has the same question as you do - the answer contains multiple links to additional ressources learning about I2C in general and notes:

[quote userid="6462" url="~/f/nordic-q-a/62384/how-does-nrf_drv_twi_tx-write-to-16-bit-register-address/254197#254197"]

Note that I2C itself has no notion of "registers" - that's just to do with the particular Slave you're talking to.

As far as I2C itself is concerned, it's all just data.

[/quote]

[quote userid="79066" url="~/f/nordic-q-a/87300/nrf52832-twi-implementation-with-16-bit-register-addresses"]functions seem to be limited to 8-bit addresses[/quote]

Do you mean the I2C device address? I think the I2C protocol specifies 7 bit standard addresses and 10 bit addressing to expand the number of devices connected to the same bus.

I assume you confuse the I2C device address with the "register address".

The datasheet of the Melexis MLX90632 states (on p.22):

"By default, the device responds to the 7-bit slave address 0x3A.
Configuration of the 7-bit slave address is possible at EEPROM address 0x24D5."

Section 9.2 Addressed read of the datasheet (p.24) shows how to perform a read from a 16-bit address on the device.
1. Send the 7-bit device address
2. Send the MSB of the 16 bit address
3. Send the LSB of the 16 bit address

JaTi