Implementing ACK in software ESB

Question

Hello, 
 I have to implement ESB on nRF52 bare metal (using the RADIO peripheral). I can receive packets from the nRF24, but I need to implement the ACK, and I'm not sure what this looks like. From the nrf24l01+ datasheet, I know a packet has a preamble, address, packet control field, payload and CRC. 
 I know that the radio stores the S0, length, S1 and payload fields in memory. So how would I set these fields for the ACK? 
 In this case, for ESB, there is no S0, the length is 6 bits, S1 is 3 bits (2 bit PID and 1 bit NOACK). I do not need to send a payload back yet, although I would appreciate an explanation of how the packets would be different in the event that I returned a payload with the ACK. 
 What should I set these fields to for the ACK? And what about the CRC? 
 Best

bjorn-spockeli · Accepted Answer

Hi Cwmoreiras,

just out of curiosity, why can't you use the ESB implementation in our SDK? It is backwards compatible with the nRF24, see Backward compatibility

Looking at our ESB implementation I found the following functions

static void update_rf_payload_format_esb_dpl(uint32_t payload_length)
{
#if (NRF_ESB_MAX_PAYLOAD_LENGTH <= 32)
    // Using 6 bits for length
    NRF_RADIO->PCNF0 = (0 << RADIO_PCNF0_S0LEN_Pos) |
                       (6 << RADIO_PCNF0_LFLEN_Pos) |
                       (3 << RADIO_PCNF0_S1LEN_Pos) ;
#else
    // Using 8 bits for length
    NRF_RADIO->PCNF0 = (0 << RADIO_PCNF0_S0LEN_Pos) |
                       (8 << RADIO_PCNF0_LFLEN_Pos) |
                       (3 << RADIO_PCNF0_S1LEN_Pos) ;
#endif
    NRF_RADIO->PCNF1 = (RADIO_PCNF1_WHITEEN_Disabled    << RADIO_PCNF1_WHITEEN_Pos) |
                       (RADIO_PCNF1_ENDIAN_Big          << RADIO_PCNF1_ENDIAN_Pos)  |
                       ((m_esb_addr.addr_length - 1)    << RADIO_PCNF1_BALEN_Pos)   |
                       (0                               << RADIO_PCNF1_STATLEN_Pos) |
                       (NRF_ESB_MAX_PAYLOAD_LENGTH      << RADIO_PCNF1_MAXLEN_Pos);
}


static void update_rf_payload_format_esb(uint32_t payload_length)
{
    NRF_RADIO->PCNF0 = (1 << RADIO_PCNF0_S0LEN_Pos) |
                       (0 << RADIO_PCNF0_LFLEN_Pos) |
                       (1 << RADIO_PCNF0_S1LEN_Pos);

    NRF_RADIO->PCNF1 = (RADIO_PCNF1_WHITEEN_Disabled    << RADIO_PCNF1_WHITEEN_Pos) |
                       (RADIO_PCNF1_ENDIAN_Big          << RADIO_PCNF1_ENDIAN_Pos)  |
                       ((m_esb_addr.addr_length - 1)    << RADIO_PCNF1_BALEN_Pos)   |
                       (payload_length                  << RADIO_PCNF1_STATLEN_Pos) |
                       (payload_length                  << RADIO_PCNF1_MAXLEN_Pos);
}

Also looking at the NRF_ESB_PROTOCOL_ESB_DPL case in start_tx_transaction() might offer some clues.

static void start_tx_transaction()
{
    bool ack;

    m_last_tx_attempts = 1;
    // Prepare the payload
    mp_current_payload = m_tx_fifo.p_payload[m_tx_fifo.exit_point];


    switch (m_config_local.protocol)
    {
        case NRF_ESB_PROTOCOL_ESB:
            update_rf_payload_format(mp_current_payload->length);
            m_tx_payload_buffer[0] = mp_current_payload->pid;
            m_tx_payload_buffer[1] = 0;
            memcpy(&m_tx_payload_buffer[2], mp_current_payload->data, mp_current_payload->length);

            NRF_RADIO->SHORTS   = m_radio_shorts_common | RADIO_SHORTS_DISABLED_RXEN_Msk;
            NRF_RADIO->INTENSET = RADIO_INTENSET_DISABLED_Msk | RADIO_INTENSET_READY_Msk;

            // Configure the retransmit counter
            m_retransmits_remaining = m_config_local.retransmit_count;
            on_radio_disabled = on_radio_disabled_tx;
            m_nrf_esb_mainstate = NRF_ESB_STATE_PTX_TX_ACK;
            break;

        case NRF_ESB_PROTOCOL_ESB_DPL:
            ack = !mp_current_payload->noack || !m_config_local.selective_auto_ack;
            m_tx_payload_buffer[0] = mp_current_payload->length;
            m_tx_payload_buffer[1] = mp_current_payload->pid << 1;
            m_tx_payload_buffer[1] |= mp_current_payload->noack ? 0x00 : 0x01;
            memcpy(&m_tx_payload_buffer[2], mp_current_payload->data, mp_current_payload->length);

            // Handling ack if noack is set to false or if selective auto ack is turned off
            if (ack)
            {
                NRF_RADIO->SHORTS   = m_radio_shorts_common | RADIO_SHORTS_DISABLED_RXEN_Msk;
                NRF_RADIO->INTENSET = RADIO_INTENSET_DISABLED_Msk | RADIO_INTENSET_READY_Msk;

                // Configure the retransmit counter
                m_retransmits_remaining = m_config_local.retransmit_count;
                on_radio_disabled = on_radio_disabled_tx;
                m_nrf_esb_mainstate = NRF_ESB_STATE_PTX_TX_ACK;
            }
            else
            {
                NRF_RADIO->SHORTS   = m_radio_shorts_common;
                NRF_RADIO->INTENSET = RADIO_INTENSET_DISABLED_Msk;
                on_radio_disabled   = on_radio_disabled_tx_noack;
                m_nrf_esb_mainstate = NRF_ESB_STATE_PTX_TX;
            }
            break;

        default:
            // Should not be reached
            break;
    }

    NRF_RADIO->TXADDRESS    = mp_current_payload->pipe;
    NRF_RADIO->RXADDRESSES  = 1 << mp_current_payload->pipe;

    NRF_RADIO->FREQUENCY    = m_esb_addr.rf_channel;
    NRF_RADIO->PACKETPTR    = (uint32_t)m_tx_payload_buffer;

    NVIC_ClearPendingIRQ(RADIO_IRQn);
    NVIC_EnableIRQ(RADIO_IRQn);

    NRF_RADIO->EVENTS_ADDRESS = 0;
    NRF_RADIO->EVENTS_PAYLOAD = 0;
    NRF_RADIO->EVENTS_DISABLED = 0;

    DEBUG_PIN_SET(DEBUGPIN4);
    NRF_RADIO->TASKS_TXEN  = 1;
}

Best regards

Bjørn

Implementing ACK in software ESB

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