MCP7940 drivers for nrf52840

[Vishal Aditya]

We are working on external RTC MCP7940(i2c) interfaced with nrf52840. Please check if drivers of the same for nrf52/51 are available or any implementation of CLOCK using MCP7940 with NRF?

Regards

Vishal Aditya

Embedded Software Engineer

[bjorn-spockeli]

HI Vishal, 

I am afraid that we do not have any dedicated drivers from the MCP7940. I am also not aware of any customers that have used the  MCP7940  as the 32kHz clock source for the nRF5x series, but  Ithink it should be possible. 

We have TWI hardware drivers in our nRF5 SDK for the nRF51 and 52 series, see Driver support matrix. 

Best regards

Bjørn

[Vishal Aditya]

bjorn-spockeli
I am writing drivers by myself & stuck in the first step of I2C DETECT not happening.

SDK: nRF5_SDK_15.2.0_9412b96\examples\peripheral\twi_scanner
MCP7940 Break-out: https://rheingoldheavy.com/product/breakout-board-mcp7940/ which works perfectly with Arduino at I2C Address: 0x6F

But I2C DETECT not working with nrf52840 DK. Please suggest how to debug this issue? We have an Oscilloscope!

[bjorn-spockeli]

If you want to start the oscillator, then you need to set bit 7 in REG_RTCSEC to 1, i.e. you need to write 0x80 not 0x07. 

The read operation is Not Acknowledged so it could be that you have to select the register you want to read from before 

// Read REG_RTCSEC register

uint8_t tx_data[] = {REG_RTCSEC };
uint8_t rx_data[] = {0};

// Select REG_RTCSEC register and use repeated start
err_code = nrf_drv_twi_tx(&m_twi,MCP7940_I2C,tx_data , sizeof(tx_data),true);
APP_ERROR_CHECK(err_code);

// Read REG_RTCSEC register 
err_code = nrf_drv_twi_rx(&m_twi,MCP7940_I2C,rx_data,1);
APP_ERROR_CHECK(err_code);

[Vishal Aditya]

Please check still NAK

[bjorn-spockeli]

Yes, but you are reading back the value of the register, i.e. 0x07. Wasnt that what you wrote to the register previously? What is the return code of nrf_drv_twi_rx()? Is it NRF_ERROR_DRV_TWI_ERR_DNACK or NRF_ERROR_DRV_TWI_ERR_ANACK?

[Vishal Aditya]

err_code = 0

[bjorn-spockeli]

Ok, if the nrf_drv_twi_rx call does not return a non-zero error code, then things are working as they should. Could you try to run the following code and see if the rx_data[0] is 0x80. If it is then the code is working as intended. 

// Initizalize buffers 	
uint8_t tx_data[] = {REG_RTCSEC, 0x80 }; // Set TX buffer - Write 0x80 ( Enable Osc) to REG_RTCSEC register
uint8_t rx_data[] = {}; // Empty RX buffer

// Write Tx data to MCP7940
err_code = nrf_drv_twi_tx(&m_twi,MCP7940_I2C,tx_data , sizeof(tx_data),false);
APP_ERROR_CHECK(err_code);

// Select REG_RTCSEC register and use repeated start (only send 1 byte of tx buffer)
err_code = nrf_drv_twi_tx(&m_twi,MCP7940_I2C,tx_data , 1 ,true);
APP_ERROR_CHECK(err_code);

// Read REG_RTCSEC register 
err_code = nrf_drv_twi_rx(&m_twi,MCP7940_I2C,rx_data,1);
APP_ERROR_CHECK(err_code);

Best regards

Bjørn

[Vishal Aditya]

Yes getting 0x80

Below is the attached Arduino init sequence:

  Wire.begin();                               // Start I2C as master device
  Wire.setClock(i2cSpeed);                    // Set the I2C bus speed
  Wire.beginTransmission(MCP7940_ADDRESS);    // Address the MCP7940M
  uint8_t errorCode = Wire.endTransmission(); // See if there's a device present
  if (errorCode == 0)                         // If we have a MCP7940M
  {
    clearRegisterBit(MCP7940_RTCHOUR, MCP7940_12_24);                      // Use 24 hour clock
    setRegisterBit(MCP7940_CONTROL, MCP7940_ALMPOL);                       // assert alarm low, default high
    _CrystalStatus = readRegisterBit(MCP7940_RTCSEC, MCP7940_ST);          // Status bit from register
    _OscillatorStatus = readRegisterBit(MCP7940_RTCWKDAY, MCP7940_OSCRUN); // Oscillator state from register
  }

MCP7940.h

MCP7940.cpp

[bjorn-spockeli]

Great, then you are good to go. You should be able to reuse the snippet I provided to configure the MCP7940.  My suggestion would be to capture a trace of the Arduino Init sequence to see which values that are written to which register and then just copy paste the code snippet I provided and change the REG_RTCSEC register value and the 0x80 to match what you're seeing in the trace.