nrf_drv_twi dynamically adjust receiving length

[joe.ker]

Hi,

I am porting a NCI (Nfc Controller Interface) driver to the nRF52. One of the things I am struggling with is that, when reading from the device, the amount of data to be read is part of the first 3 bytes. Is it possible to set up a transfer of 250 bytes and the, after reading the first 3 bytes, downsizing to the right amount of bytes (which is part of these first 3 bytes) without interrupting the transfer?

Thanks and regards, Josef

[endnode]

Hi, isn't TWI (I2C) read managed by slave ACK/NACK so by putting maximum buffer size (and pointer;) you will read all bytes available up to Slave's NACK (or buffer overflow) and you will then read actual number of bytes read upon NRF_DRV_TWI_EVT_DONE event from nrf_drv_twi_evt_t structure (including data stream itself)?

[Phil Young]

If the slave has no data available to send it sends nothing resulting in clock extension by the slave until the I2C times out and the master aborts the transaction, so this would not work.

[Phil Young]

It appears that it is possible to run the TWI in the legacy ( deprecated ) mode on the NRF52 so it should be OK using the original NRF51 code. For some reason Nordic now appears to have 3 separate I2C interface options, 2 using EZDMA ( 1 master and 1 slave ), and 1 like the NRF51 using interrupts, in reality they are 1 peripheral with 3 modes but Nordic could not even bother to keep them together in the datasheet so it's easy to not notice this point. As usual, the Nordic HW is good but let down by the poor documentation.

[endnode]

Then this is not how normal I2C works, there should be no time-out if you use clock stretching (or there could be some sort of watch-dog but then both sides need to be ready for certain recovery procedure to set I2C to proper initial state = all HIGH and continue with address sending/reception again).

[endnode]

Btw. how this relates to the original question where the problem was "how to find out that I2C transfer ended with less bytes received then buffer length"?