MMA8652FC & Transients

[haroldmccabe]

This accelerometer on my prototype board detects transient interrupts OK. But when I read the transient source reg (0x1E) I often see 0x00; at other times I see the event flag and other axis/polarity bits set as expected.

Why do I get a transient interrupt but the transient source reg = 0x00?

I tried setting the ELE bit in the transient config reg (0x1D) but then, once I get a transient interrupt I cannot clear it and the accelerometer interrupts continually.

My initialization is as follows:

// Reset and pause at least 1 ms

SMB_Write(&accel_write_reset); // reg 0x2B <- 0x40

stop_time = (U8)ticks + 2;

while ((U8)ticks < stop_time) ;

// Transient configuration register

SMB_Write(&accel_write_trans_cfg_reg); // reg 0x1D <- 0x1E

// Transient threshold register

SMB_Write(&accel_write_trans_thresh_reg); // reg 0x1F <- 0x90

// Time threshold (width) of transient

SMB_Write(&accel_write_trans_count_reg); // reg 0x20 <- 0x01

// Use high-pass filter

SMB_Write(&accel_write_xyz_data_config); // reg 0x0E <- 0x10

SMB_Write(&accel_write_hpf);

// Configure interrupt as open drain

SMB_Write(&accel_write_control_reg_3); // reg 0x2C <- 0x01

// Write portrait/landscape config

SMB_Write(&accel_write_pl_config); // reg 0x11 <- 0x40

// Write debounce count

SMB_Write(&accel_write_pl_count); // reg 0x12 <- 0xFF

// Take defaults for PL_BF_ZCOMP and PL_THS_REG

// Route interrupts

SMB_Write(&accel_write_control_reg_5); // reg 0x2E <- 0x30

// Enable interrupts

SMB_Write(&accel_write_control_reg_4); // reg 0x2D <- 0x30

// Write to control register 1: active, ODR, fast read mode

SMB_Write(&accel_write_control_reg_1); // reg 0x2A <- 0x0B

[TomasVaverka · Accepted solution]

Hi Harold,

It looks like you have both transient and orientation interrupts enabled and routed to the same INT1 pin. Are you reading first the INT_SOURCE register to distinguish between the transient and orientation interrupt and then only an appropriate event source register (either PL_STATUS or TRANSIENT_SRC) as illustrated below?

IntSource = I2C_ReadRegister(MMA845x_I2C_ADDRESS, INT_SOURCE_REG);

if (IntSource & 0x20)      // Transient interrupt?

{

TransientSource = I2C_ReadRegister(MMA865x_I2C_ADDRESS, TRANSIENT_SRC_REG);     // Service and clear the transient interrupt 

}

if (IntSource & 0x10)      // Orientation interrupt?

{

PLStatus = I2C_ReadRegister(MMA865x_I2C_ADDRESS, PL_STATUS_SRC);      // Service and clear the orientation interrupt 

}

Regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct" or “Helpful”. Thank you.

[haroldmccabe]

Yes, both transient and orientation interrupts were enabled and going to the same pin.

I found the ELE bit in the transient config reg must be set to latch the transient axis/polarity info into the transient source reg. Otherwise the axis/polarity info may change between when the transient interrupt is asserted and when the transient source reg is read.

Earlier, I was not actually getting continual interrupts with ELE set: I was mislead by my debug setup.

Thank you, Tomas.