Negative TX power bug in ble_dtm.c

[Jimmie Johansson]

As far as I can tell there is a sign error bug when setting the tx power to a negative value. ble_dtm.c uses an int32_t to represent the tx power so when calling dtm_set_txpower with -4 (0xFC) the resulting bits in m_tx_power will be 0xFFFFFFFC.

Later when dtm_init() is called, m_tx_power is validated in ble_dtm_hw_nrf51.c and checked against the valid values in nrf51_bitfields.h, all of which are defined as (0xXXUL). This comparison will always fail and dtm_init reports invalid configuration.

It can be solved by adding the following line of code in dtm_set_txpower:

m_tx_power &= 0xFF;

Am I missing something here? Is there another way that this is supposed to work?

[Jørgen Holmefjord]

What SDK version are you using? There was a fix for negative TX power values in the DTM library in SDK 14.2. From your description it looks like you are using nRF51, which indicate you might be using an older version of the SDK.

[Jimmie Johansson]

I am indeed using an older version (12.3.0), but I have looked at ble_dtm.c in SDK 14.2 and the code related code looks exactly the same (int32_t m_tx_power, validation against unsigned long values and casting from int8_t to int32_t which fills the msb of m_tx_power with FF).
I have changed my code to always set tx power to 0 before I call dtm_init() and then resetting it to the negative value. That works, but the RADIO->TX_POWER will still be set with 0xFFFFFFFC (-4) even though the reference manual kind of states that it should be set to 0x000000FC.