nrf_twi_mngr handling of transfers larger than 255 Bytes

RE: nrf_twi_mngr handling of transfers larger than 255 Bytes

Daniel Veilleux — Thu, 19 Jul 2018 18:09:45 GMT

It looks like the NRF_TWI_MNGR_NO_STOP flag just enables a repeated start instead of a stop followed by a start. From nrf_twi_mngr.h:

* Use this flag when a stop condition is undesirable between two transfers,
* for example, when the first transfer is a write that sets an address in the slave
* device and the second one is a read that fetches certain data using this
* address. In this case, the second transfer should follow directly after the
* first transfer, with a repeated start condition instead of a stop and then
* a new start condition.

However, I spent a few minutes looking at nrfx_twi.c and it looks like twi_send_byte executes the NRF_TWI_TASK_SUSPEND task when it gets to the end of the first data buffer and NRF_TWI_MNGR_NO_STOP is set (instead of executing NRF_TWI_TASK_STOP). Then twi_tx_start_transfer is called to send the second buffer and it's in here that the NRF_TWI_TASK_RESUME and NRF_TWI_TASK_STARTTX tasks are executed. I think the NRF_TWI_TASK_STARTTX is the task that actually puts the repeated start condition and address on the bus again. To test this I used a static variable to remember if the TWI bus was previously suspended instead of stopped -- if so I allow twi_tx_start_transfer to execute the NRF_TWI_TASK_RESUME task but skip NRF_TWI_TASK_STARTTX . The result on my logic analyzer is start, address with write bit, 255 bytes, 255 more bytes, and then stop.

I didn't have time to test if this breaks something else in the driver but if I understand your question correctly this should at least prove that it's possible to do what you're looking for.

devzone.nordicsemi.com/.../nrfx_5F00_twi.c

RE: nrf_twi_mngr handling of transfers larger than 255 Bytes

raemond — Fri, 13 Jul 2018 20:07:55 GMT

I noticed the limitation on the length set by the usage of a uint8_t buffer length variable. So that's why I broke it up into pieces. Comments with the twi_mngr indicated that they would be grouped if I used the NRF_TWI_MNGR_NO_STOP flag, but that doesn't appear to be the case.

RE: nrf_twi_mngr handling of transfers larger than 255 Bytes

jakub — Fri, 13 Jul 2018 19:54:09 GMT

I think I know what the problem is, twi_mngr is using legacy driver. Legacy driver has hard coded limitation set on transmit buffers to 8bits.

One needs to migrate twi_mngr to use nrfx driver and the problem will be solved. Nrfx driver is capable to use tx buffer according to used SoC:

8 bit for nrf52832
10 bit for nrf52810
16 bit for nrf52840

RE: nrf_twi_mngr handling of transfers larger than 255 Bytes

raemond — Fri, 13 Jul 2018 19:44:20 GMT

It seems to happen whether or not EasyDMA is enabled.

RE: nrf_twi_mngr handling of transfers larger than 255 Bytes

raemond — Fri, 13 Jul 2018 16:57:19 GMT

1. SDK 15.0.0

2. nRF52840

3. EasyDMA is enabled for the interface.

RE: nrf_twi_mngr handling of transfers larger than 255 Bytes

Jakub Rzeszutko — Fri, 13 Jul 2018 05:46:01 GMT

Hi,

1. What SDK are you using?

2. What uC are you using?

3. Do you use I2C with EasyDMA support?