repeated start for twi master rx operation

[George Leung]

Hi,

I am using nRF52 as a twi mater to read some data from a touch sensor, I want to perform below operation:

twi start

twi rx register_a for few bytes

repeated start

twi rx register_c for few bytes

repeated start

... ...

twi rx register_x for few bytes

twi stop

I am using nrf_drv_twi.c in the nRF5_SDK_11.0. It seems possible for repeated start in tx case but not possible in rx case.

Is it possible to work on that? Thanks

[Keton]

if register X means that you have to send some opcode to the I2C slave then you are actually making a write operation so just use TX_RX xfer in either app_twi of nrf_drv_twi_xfer

[George Leung]

Let's me provide some more detail about the operation, the sequence should be that: Start Addr+W ACK registerA ACK Start Addr+R ACK data1 ACK datan NACK Start Addr+W ACK registerC ACK Start Addr+R ACK data1 ACK data...n NACK .... Start Addr+W ACK registerX ACK Start Addr+R ACK data1 ACK datan NACK Stop The major issue is I don't want to send a stop when last byte of current register read. Is it possible?

[Keton]

I2C protocol doesn't have a concept of registers. All bytes you transmit are raw data and peripheral interprets them according to its specification. Unless your I2C device is very weird and uncommon, You really want to do following seq:

TX: register 1 OPcode
Repeated start
RX: register 1 contents
STOP
repeat for register 2

for info how to do it consult infocenter.nordicsemi.com/.../hardware_driver_twi.html and use either blocking calls of nrf_drv_twi_tx with no_stop set to true + nrf_drv_twi_rx or async NRF_DRV_TWI_XFER_TXRX. There are SDK examples which show this in more detail.

[George Leung]

don't care too much about the term "register", it just one byte memory address of the touch sensor which I am going to read. nrf_drv_twi_tx with no_stop is good but how about nrf_drv_twi_rx? I don't want any stop between each read operation. As I send a stop the sensor chip will perform next measurement which is not desire.

[Keton]

As far as I know SDK doesn't support "no stop RX". Also I haven't seen use case for such feature in real life and I do quite a lot of I2C peripheral drivers. Are you sure that no stop condition after rx is really what you want?