What exactly do nrf_drv_twi_disable() and _enable() do?

[Ladasky]

I am working with a sensor which has a programmable configuration register. After changing this register, I have to reboot the sensor. To do this, I need to temporarily suspend a TWI master driver and manipulate the SCL and SDA GPIO lines directly, as described in this earlier discussion:

devzone.nordicsemi.com/.../

Looking through the Nordic documentation, I'm trying to figure out how to get this done.

In the SDK docs I have found the functions nrf_drv_twi_disable() and nrf_drv_twi_enable(). These functions are sparsely documented. Nor are these functions used in any of the software examples. What exactly do they do?

In the hardware docs (Section 47) I found references to the registers PSELSCL and PSELSDA. These registers determine the pins that are driven by the TWI. I found these elements within the NRF_TWI_Type data structure, and I can read their values. The documentation says I can only WRITE these registers when the TWI is disabled.

I started with a pretty simple modification to an existing program; just adding calls to nrf_drv_twi_disable() and nrf_drv_twi_enable() after some data transfers have completed, with short delays in between each call (0.5 seconds each). I haven't tried to modify PSELSCL or PSELSDA yet, or to write to the GPIO lines. Still, this simple change crashes. APP_ERROR_CHECK fails, my program reboots, and then the first new thing that my program tries to do on the TWI (just reading a register) causes another APP_ERROR_CHECK failure.

I don't know that any of this is Nordic's fault, since the slave device is obviously a little strange. But the failure is profound. When my system gets into an error state, I can only solve the problem by completely unplugging everything. I have to turn off my nRF52 board, and disconnect it from USB. I even have to unplug my slave device from the Nordic board (which is also its power source).

After I reboot, provided that a working program is in flash, everything functions again. By "working program" I mean that one which changes the configuration register, but doesn't (yet) perform the sleep-and-wake cycle that I want.

I am using a the nRF52 Preview board and SDK 0.9.2 (the highest revision that I can use).

Thanks for any help you can provide!

[Ladasky]

Getting error codes has sometimes been frustrating. I am using UART to observe the behavior of my device. If I don't include calls to fflush() and nrf_delay_ms() after every printf(), part or all of the error message that I want to read will fail to print before APP_ERROR_CHECK reboots the nRF52.

Still, what I've seen is that, when an error actually prints, I get error code 3. I went hunting through the source code and error code 3 is NRF_ERROR_INTERNAL, which is generated "if an NRF_TWI_EVENTS_ERROR or a time-out has occurred (only in blocking mode)." That doesn't tell me much. I have to dig deeper.

I may just take a hardware approach to solve my problem. I might route the power supply of my sensor through an analog switch, and control that switch with a GPIO. That would prevent me from having to figure out the problems I'm having with suspending Nordic's TWI.