TWI

Software/Hardware controlled TWI Master driver.

Features supported

• Repeated start
• No multi-master
• Only 7-bit addressing
• Supports clock stretching (with optional SMBus style slave timeout using software driver)
• Tries to handle slaves stuck in the middle of transfer

Documentation specific to HW controlled TWI master driver.

Code snippet below illustrates the write sequence of the HW controlled TWI master driver.

    while (true)
    {
        while (NRF_TWI1->EVENTS_TXDSENT == 0 && NRF_TWI1->EVENTS_ERROR == 0 && (--timeout))
        {
            // Do nothing.
        }

        if (timeout == 0 || NRF_TWI1->EVENTS_ERROR != 0)
        {
            // Recover the peripheral as indicated by PAN 56: "TWI: TWI module lock-up." found at
            // Product Anomaly Notification document found at 
            // https://www.nordicsemi.com/eng/Products/Bluetooth-R-low-energy/nRF51822/#Downloads
            NRF_TWI1->EVENTS_ERROR = 0;
            NRF_TWI1->ENABLE       = TWI_ENABLE_ENABLE_Disabled << TWI_ENABLE_ENABLE_Pos; 
            NRF_TWI1->POWER        = 0;
            nrf_delay_us(5);
            NRF_TWI1->POWER        = 1;
            NRF_TWI1->ENABLE       = TWI_ENABLE_ENABLE_Enabled << TWI_ENABLE_ENABLE_Pos;

            (void)twi_master_init();

            return false;
        }
        NRF_TWI1->EVENTS_TXDSENT = 0;
        if (--data_length == 0)
        {
            break;
        }

        NRF_TWI1->TXD = *data++;
    }

Code snippet below illustrates the read sequence of the HW controlled TWI master driver.

    while (true)
    {
        while (NRF_TWI1->EVENTS_RXDREADY == 0 && NRF_TWI1->EVENTS_ERROR == 0 && (--timeout))
        {
            // Do nothing.
        }
        NRF_TWI1->EVENTS_RXDREADY = 0;

        if (timeout == 0 || NRF_TWI1->EVENTS_ERROR != 0)
        {
            // Recover the peripheral as indicated by PAN 56: "TWI: TWI module lock-up." found at
            // Product Anomaly Notification document found at
            // https://www.nordicsemi.com/eng/Products/Bluetooth-R-low-energy/nRF51822/#Downloads
            NRF_TWI1->EVENTS_ERROR = 0;
            NRF_TWI1->ENABLE       = TWI_ENABLE_ENABLE_Disabled << TWI_ENABLE_ENABLE_Pos;
            NRF_TWI1->POWER        = 0;
            nrf_delay_us(5);
            NRF_TWI1->POWER        = 1;
            NRF_TWI1->ENABLE       = TWI_ENABLE_ENABLE_Enabled << TWI_ENABLE_ENABLE_Pos;

            (void)twi_master_init();

            return false;
        }

        *data++ = NRF_TWI1->RXD;

        /* Configure PPI to stop TWI master before we get last BB event */
        if (--data_length == 1)
        {
            NRF_PPI->CH[0].TEP = (uint32_t)&NRF_TWI1->TASKS_STOP;
        }

        if (data_length == 0)
        {
            break;
        }

        // Recover the peripheral as indicated by PAN 56: "TWI: TWI module lock-up." found at
        // Product Anomaly Notification document found at
        // https://www.nordicsemi.com/eng/Products/Bluetooth-R-low-energy/nRF51822/#Downloads
        nrf_delay_us(20);
        NRF_TWI1->TASKS_RESUME = 1;
    }

Documentation specific to SW controlled TWI master driver.

Code snippet below illustrates the write sequence of the SW controlled TWI master driver.

    // Make sure SDA is an output
    TWI_SDA_OUTPUT();

    // MSB first
    for (uint_fast8_t i = 0x80; i != 0; i >>= 1)
    {
        TWI_SCL_LOW();
        TWI_DELAY();

        if (databyte & i)
        {
            TWI_SDA_HIGH();
        }
        else
        {
            TWI_SDA_LOW();
        }

        if (!twi_master_wait_while_scl_low())
        {
            transfer_succeeded = false; // Timeout
            break;
        }
    }

    // Finish last data bit by pulling SCL low
    TWI_SCL_LOW();
    TWI_DELAY();

Code snippet below illustrates the read sequence of the SW controlled TWI master driver.

    // Make sure SDA is an input
    TWI_SDA_INPUT();

    // SCL state is guaranteed to be high here

    // MSB first
    for (uint_fast8_t i = 0x80; i != 0; i >>= 1)
    {
        if (!twi_master_wait_while_scl_low())
        {
            transfer_succeeded = false;
            break;
        }

        if (TWI_SDA_READ())
        {
            byte_read |= i;
        }
        else
        {
            // No need to do anything
        }

        TWI_SCL_LOW();
        TWI_DELAY();
    }

    // Make sure SDA is an output before we exit the function
    TWI_SDA_OUTPUT();