APP_UART_FLOW_CONTROL_LOW_POWER missing on SDK11?

[Jim Dattolo]

I have been using this mode to init the app_uart_fifo code on SDK8 for a while now and when moving over to the SDK11 code I was in the app_uart_fifo.c code and it's radically different than stack 8. All of the low power code and gpiote code to catch the CTS transitions are gone. It's still listed in the header file as an option, however no code uses it that I can see.

Did something else change to manage the 1mA continuous current draw issue when using the UART? if it's been solved a different way (e.g. not using the 16mhz clock for the UART) then it would be nice to have it documented. I see nothing in the migration document to say that the UART is being handled any differently with respect to the APIs.

[Christopher]

The UART module of the nRF51 chip still require the HFCLK (see the 8.3 Block resource requirements chapter in the nRF51822 Product Specification v3.1). The UART 1mA continuous current draw is an hardware issue, not a software issue.

From SDK 10, an Hardware Abstraction Layer (HAL) and low-level drivers have been added. The UART peripheral code has changed a lot, but the same features are available (for instance the APP_UART_FLOW_CONTROL_LOW_POWER mode). See the UART module documentation.

[Jim Dattolo]

nope it's not there any more. The entire app_uart_fifo module code to manage the APP_UART_FLOW_CONTROL_LOW_POWER has been removed and it only sends on NRF_UART_HWFC_DISABLED and NRF_UART_HWFC_ENABLED down to the low level code but it only ever checks the enum passed against APP_UART_FLOW_CONTROL_DISABLED. The only thing left is the enum that defines the low power state, greping for it in the components dir of the SDK finds nothing uses it.

[Jim Dattolo]

verified that the code exists on SDK8 and 9, missing on 10 and 11 grep shows only an error case in app_uart stating that it's not supported, no ref in the fifo code or any uart driver code.

[Susheel Nuguru]

seems like the support is removed and not documented in uart library. This could most probably because of the power impact using gpiote (not sure) but I have created an internal ticket to update the documentation and/or probably remove that mode.

[Jim Dattolo]

removal of the low power support makes the chip unusable on battery based applications that need the UART. The entire point of the low power mode was to allow apps that could do the non-standard flow control the ability to keep the UART off and save the 1mA continuous draw. By formally removing the support you are forcing your customers to either keep SDK9 running or if they upgrade they need to copy the SDK9 UART code into their app and use that. Hardly a good solution if someone is just starting out with the chip and never was on SDK8 or 9

[Sydney]

I'm in the process of migrating from SDK7 to SDK11 and started realizing all the low power UART code appeared to be missing, even though the documentation for app_uart_flow_control_t clearly says, "Flow control setting, if flow control is used, the system will use low power UART mode, based on CTS signal." This makes it sound like any use of flow control will use the low power, CTL-controlled implementation, but I don't see the code to do that anywhere.