samples: bluetooth: DTM (FEM) fix assert when setting tx power by NCD

Fix assertion in the direct_test_mode sample working with FEM, where
after setting the Tx power from the nRF Connect for Desktop app
the FW would try setting power through FEM API passing the encoded
power value as the "channel number" argument.
The vendor-specific command used by the app uses the "channel" field
for passing the parameter value and the FW incorrectly recorded that
value in it's static data.
Additionally clarified available power values in the FEM section of
the sample README.rst file and removed unused defines.

Ref. NCSDK-34026

Signed-off-by: Michał Grochala <[email protected]>

Author: grochu

samples/bluetooth/direct_test_mode/README.rst

@@ -284,7 +284,7 @@ Vendor-specific commands can be divided into different categories as follows:
 * If the **Length** field is set to ``1`` (symbol ``CARRIER_TEST_STUDIO``), the field value is used by the nRFgo Studio to indicate that an unmodulated carrier is turned on at the channel.
   It remains turned on until a ``TEST_END`` or ``RESET`` command is issued.
 * If the **Length** field is set ``2`` (symbol ``SET_TX_POWER``), the **Frequency** field sets the TX power in dBm.
-  The valid TX power values are specified in the product specification ranging from –40 to +4, where ``0`` dBm is the reset value.
+  The valid TX power values are specified in the product specification of the SoC used, where ``0`` dBm is the reset value.
   Only the six least significant bits will fit in the **Length** field.
   The two most significant bits are calculated by the DTM module.
   This is possible because the six least significant bits of all valid TX power values are unique.

samples/bluetooth/direct_test_mode/src/dtm.c

@@ -184,10 +184,6 @@ BUILD_ASSERT(NRFX_TIMER_CONFIG_LABEL(ANOMALY_172_TIMER_INSTANCE) == 1,
 #define PACKET_BA_LEN             3
 /* CTE IQ sample data size. */
 #define DTM_CTE_SAMPLE_DATA_SIZE  0x52
-/* Vendor specific packet type for internal use. */
-#define DTM_PKT_TYPE_VENDORSPECIFIC  0xFE
-/* 1111111 bit pattern packet type for internal use. */
-#define DTM_PKT_TYPE_0xFF            0xFF
 
 /* Maximum number of payload octets that the local Controller supports for
  * transmission of a single Link Layer Data Physical Channel PDU.
@@ -1890,8 +1886,10 @@ static int dtm_vendor_specific_pkt(uint32_t vendor_cmd, uint32_t vendor_option)
 		}
 
 		/* Not a packet type, but used to indicate that a continuous
-		 * carrier signal should be transmitted by the radio.
+		 * carrier signal should be transmitted by the radio on the
+		 * channel indicated by the 'option' field.
 		 */
+		dtm_inst.phys_ch = vendor_option;
 		radio_prepare(TX_MODE);
 		nrf_radio_fast_ramp_up_enable_set(NRF_RADIO, IS_ENABLED(CONFIG_DTM_FAST_RAMP_UP));
 
@@ -2402,9 +2400,15 @@ int dtm_test_transmit(uint8_t channel, uint8_t length, enum dtm_packet pkt)
 
 	dtm_inst.packet_type = pkt;
 	dtm_inst.packet_len = length;
-	dtm_inst.phys_ch = channel;
 	dtm_inst.current_pdu = dtm_inst.pdu;
 
+	/* 'channel' may be used for different purposes if the packet
+	 * is vendor-specific
+	 */
+	if (pkt != DTM_PACKET_VENDOR) {
+		dtm_inst.phys_ch = channel;
+	}
+
 	/* Check for illegal values of m_phys_ch. Skip the check if the
 	 * packet is vendor specific.
 	 */
@@ -2417,7 +2421,7 @@ int dtm_test_transmit(uint8_t channel, uint8_t length, enum dtm_packet pkt)
 	/* Check for illegal values of packet_len. Skip the check
 	 * if the packet is vendor spesific.
 	 */
-	if (dtm_inst.packet_type != DTM_PKT_TYPE_VENDORSPECIFIC &&
+	if (dtm_inst.packet_type != DTM_PACKET_VENDOR &&
 	    dtm_inst.packet_len > DTM_PAYLOAD_MAX_SIZE) {
 		/* Parameter error */
 		return -EINVAL;