DTM Example Firmware Bug ?

Question

Hi
I'm setting up a firmware based on the DTM example to perform certification tests. 
 In the DTM example firmware I see that there is a VENDOR_SPECIFIC command option to use combined with the LE_TRANSMITTER_TEST command.
The VENDOR_SPECIFIC command option has other sub-commands and I'm interested in the SET_TX_POWER one. 
 In the main.c code there is the uart packet parsing: 
 static uint32_t dtm_cmd_put(uint16_t command)
{
 dtm_cmd_t command_code = (command >> 14) & 0x03;
 dtm_freq_t freq = (command >> 8) & 0x3F;
 uint32_t length = (command >> 2) & 0x3F;
 dtm_pkt_type_t payload = command & 0x03;

 // Check for Vendor Specific payload.
 if (payload == 0x03) 
 {
 payload = DTM_PKT_VENDORSPECIFIC;
 }
 return dtm_cmd(command_code, freq, length, payload);
}
 
 In the 
 dtm_cmd(dtm_cmd_t cmd, dtm_freq_t freq, uint32_t length, dtm_pkt_type_t payload)
 
 function there this call: 
 dtm_vendor_specific_pkt(length, freq);
 
 and the function is defined as 
 dtm_vendor_specific_pkt(uint32_t vendor_cmd, dtm_freq_t vendor_option)
 
 so, for VENDOR_SPECIFIC commands, the length field becomes the vendor_cmd and the freq field becomes the vendor_option (the type dtm_freq_t is defined as an uint32_t ). 
 For the SET_TX_POWER command there is a call to 
 dtm_set_txpower(vendor_option)
 
 and the function is like this: 
 bool dtm_set_txpower(uint32_t new_tx_power)
{
 // radio->TXPOWER register is 32 bits, low octet a signed value, upper 24 bits zeroed
 int8_t new_power8 = (int8_t)(new_tx_power & 0xFF);

 if (m_state > STATE_IDLE)
 {
 // radio must be idle to change the tx power
 return false;
 }

 if ((new_power8 > 4) || (new_power8 < -40))
 {
 // Parameter outside valid range: nRF radio is restricted to the range -40 dBm to +4 dBm
 return false;
 }

 if (new_tx_power & 0x03) 
 {
 // Parameter error: The nRF51 radio requires settings that are a multiple of 4.
 return false;
 }
 m_tx_power = new_tx_power;

 return true;
}
 
 My question is:
how the freq value (that for the SET_TX_POWER command is the new_tx_power value), can be a negative value? 
 freq is initially set as: 
 freq = (command >> 8) & 0x3F;
 
 so freq is a 6-bit value ( 00bbbbbb ) 
 An int8_t is a byte where the msb is the sign bit ( sbbbbbbb ), so the conversion in the function dtm_set_txpower(...) that is 
 int8_t new_power8 = (int8_t)(new_tx_power & 0xFF);
 
 in my opinion, never gets a negative value for new_power8 .

FormerMember · Accepted Answer

SDK 12.0.0 and later supports negative TX power: 
 /**@brief Function for configuring the output power for transmitter test.
 This function may be called directly, or through dtm_cmd() specifying
 DTM_PKT_VENDORSPECIFIC as payload, SET_TX_POWER as length, and the dBm value as frequency.
*/
bool dtm_set_txpower(uint32_t new_tx_power)
{
// radio->TXPOWER register is 32 bits, low octet a signed value, upper 24 bits zeroed
int8_t new_power8 = (int8_t)(new_tx_power & 0xFF);

// The two most significant bits are not sent in the 6 bit field of the DTM command.
// These two bits are 1's if and only if the tx_power is a negative number.
// All valid negative values have the fourth most significant bit as 1.
// All valid positive values have the fourth most significant bit as 0.
// By checking this bit, the two most significant bits can be determined.
new_power8 = (new_power8 & 0x30) != 0 ? (new_power8 | 0xC0) : new_power8;

if (m_state > STATE_IDLE)
{
 // radio must be idle to change the tx power
 return false;
}

m_tx_power = new_power8;

return true;
 
 }