TWI Will Not Return Slave Data

[Tom]

We are using a MMA8452Q accelerometer connected to an nRF51822 via TWI using the twi_hw_master. Reading the WHO AM I register is the first attempt to get the TWI running but no data is returned.

#define ACCELEROMETER_ADDRESS       0x1C           // if SA0 = 0
#define WHO_AM_I_REG                0x0D

data_array[0] = WHO_AM_I_REG;
twi_master_transfer((ACCEL_ADDRESS << 1) | !TWI_READ_BIT, data_array, 1, TWI_DONT_ISSUE_STOP);
twi_master_transfer((ACCEL_ADDRESS << 1) | TWI_READ_BIT, data_array, 1, TWI_ISSUE_STOP);

Here is the screenshot of the write command.

1. START
2. Slave Address (0x1D shifted left 1 bit)
3. WRITE bit
4. ACK (why doesn't it get closer to 0v?)
5. no STOP
6. WHO AM I register (0x0D)
7. ACK (why doesn't it get closer to 0v?)
8. STOP (why is there a STOP?)

Here is the screenshot of the read command.

1. START
2. Slave Address (0x1D shifted left 1 bit)
3. ACK (why doesn't it get closer to 0v?)
4. But then no data is returned

There is no data because NRF_TWI1->EVENTS_ERROR always equals 1 so twi_master_read() is exited before the data is transmitted. It is 0 until the command NRF_TWI1->TASKS_STARTRX = 1 is executed which sends out the first 9 clocks and Slave Address but also sets NRF_TWI1->EVENTS_ERROR = 1. During the write command the instruction TASKS_STARTTX = 1 does not set EVENTS_ERROR = 1 so the write command is not aborted.

Where do I go from here?

[RK]

You start by working out why the waveform shows 1/2 voltage, either you have an electrical issue or your scope isn't set up correctly and you're not seeing what's actually happening. Do you have the grounds on the board and accelerometer connected to each other?