DTM test w/ nAN34 examply.py

Question

Hi Sir, 
 
 For EN 300 328 V2.1.1, I reference below setting. 
 https://devzone.nordicsemi.com/f/nordic-q-a/21643/en-300-328-v2-1-1-receiver-blocking-test-setup/84959#84959 
 . 
 IC: 51822 
 PCB: nRF51-DK x 2 
 SDK: SDK_12.3.0\examples\dtm\direct_test_mode\pca10028\blank 
 
 I follow above link and assign the " RADIO_MODE_MODE_Nrf_250Kbit " to " m_radio_mode " in " ble_dtm.c " 
 # ble_dtm.c 
 static uint8_t m_radio_mode = RADIO_MODE_MODE_Nrf_250Kbit;//RADIO_MODE_MODE_Ble_1Mbit; 
 As download nAN34_v1.01.zip, I modify the COM port to examp.py, and run the command as below. 
 py -2 example.py 
 
 But I got the high packet loss as show below. 
 Any configuration I miss for 250K test? (1M link speed is OK, TX/RX PER is 0%) 
 Thank you. 
 BRs, Han

bjorn-spockeli · Accepted Answer

Hi Hanyu,

the an empty DTM test packet is 10 bytes, so adding a payload length of 10 will result in a total packet length of 20bytes. When using the 1Mbit Radio mode, the radio will use 8uS per byte, so 20byte * 8uS/byte = 160uS. However, when using the 250kbit mode, the transmission time per byte is increased by a factor of 4, i.e. 32uS per byte, so the same 20byte packet will now take 32uS/byte*20byte = 640uS to transmit.

If we look at the DTM code from SDK 12.3.0, we can see that the code assumes that 1Mbit is used as it uses the 8uS/byte factor to calculate the transmission time of a packet of a given m_packet_length.

 // Set the timer to the correct period. The delay between each packet is described in the
        // Bluetooth Core Spsification version 4.2 Vol. 6 Part F Section 4.1.6.
        if ((m_packet_length + DTM_ON_AIR_OVERHEAD_SIZE ) * 8  <= 376)
        {
            mp_timer->CC[0]       = 625;                        // 625uS with 1MHz clock to the timer
        }
        else if ((m_packet_length + DTM_ON_AIR_OVERHEAD_SIZE ) * 8  <= 1000)
        {
            mp_timer->CC[0]       = 1250;                        // 625uS with 1MHz clock to the timer
        }
        else if ((m_packet_length + DTM_ON_AIR_OVERHEAD_SIZE ) * 8  <= 1624)
        {
            mp_timer->CC[0]       = 1875;                        // 625uS with 1MHz clock to the timer
        }
        else
        {
            mp_timer->CC[0]       = 2500;                        // 625uS with 1MHz clock to the timer
        }

        // Configure PPI so that timer will activate radio every 625 us
        NRF_PPI->CH[0].EEP = (uint32_t)&mp_timer->EVENTS_COMPARE[0];
        NRF_PPI->CH[0].TEP = (uint32_t)&NRF_RADIO->TASKS_TXEN;
        NRF_PPI->CHENSET   = 0x01;
        m_state            = STATE_TRANSMITTER_TEST;

In our case the DTM code will send a packet every 625uS as the total packet length is 20byte, but each packet will take 640uS when using the 250kbit mode.

In the python code it also assumes that a packet will maximum use 625uS per packet.

	def _calculateTimeForPackage (self):
		"""Calculate how long time from start of one package to the start of next"""
		return 0.625*2

So I think if you modify the DTM code(dtm_cmd() in ble_dtm.c) as shown below to take in to account for the 4 time increase in transmit time.

 if ((m_packet_length + DTM_ON_AIR_OVERHEAD_SIZE ) * 32  <= 376)
        {
            mp_timer->CC[0]       = 625;                        // 625uS with 1MHz clock to the timer
        }
        else if ((m_packet_length + DTM_ON_AIR_OVERHEAD_SIZE ) * 32  <= 1000)
        {
            mp_timer->CC[0]       = 1250;                        // 625uS with 1MHz clock to the timer
        }
        else if ((m_packet_length + DTM_ON_AIR_OVERHEAD_SIZE ) * 32  <= 1624)
        {
            mp_timer->CC[0]       = 1875;                        // 625uS with 1MHz clock to the timer
        }
        else
        {
            mp_timer->CC[0]       = 2500;                        // 625uS with 1MHz clock to the timer
        }

and multiply the total time from one packet to the next in the python script by 2

	def _calculateTimeForPackage (self):
		"""Calculate how long time from start of one package to the start of next"""
		return 0.625*2

then I think you should get better results with the payload length set to 10 in the python script.

Best regards

Bjørn

DTM test w/ nAN34 examply.py

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