PWM with PCA9685 not able to generate pulsewidth more than 1.2ms

Question

I'm trying to generate PWM signals for a stepper motor with a PCA9685 development board and an MCU (STM32F4). I'm able to write and read all commands to the chip via I2C — and verify that everything I send is transferred correctly. I'm using the internal oscillator of the chip.

Currently, the chip is configured to 50 Hz (Register 0xFE set to value 0x90). A logic analyzer shows a repetition frequency of 49.18 Hz. When I write values from 205 to 450 to the OFF register (address 0x08 and 0x09), I’m only getting a pulse width from 1.02 ms down to 0.7658 ms (yes the direction seems to decrease with increasing value). No matter how I adjust the OFF register value, I can't seem to achieve any pulse widths greater than ~1 ms (not 1.5 ms and not 2 ms).

Has anyone worked with the PCA9685 chip before? Do you have any recommendations on how to fix this or what to check?

Datasheet for reference

Code Snipped of STM32F4:

int main(void)
{
   //initialization
   SystemClock_Config();
   GPIO_Config();
   //Reset PCA9685
   I2C_config();
   I2C_start();
   I2C_addr(0x00);
   I2C_write(0x06);
   I2C_stop();
   //Set Frequency
   I2C_start();
   I2C_addr(0x80);
   I2C_write(0xFE);
   I2C_write(0x90);
   I2C_stop();
   //Read current status
   I2C_start();
   I2C_addr(0x80);
   I2C_write(0xFE);
   I2C_start();
   I2C_addr(0x80|1);
   register_return_value = I2C_read(0);
   I2C_stop();
   //Read current status
   I2C_start();
   I2C_addr(0x80);
   I2C_write(0x00);
   I2C_start();
   I2C_addr(0x80|1);
   register_return_value = I2C_read(0);
   I2C_stop();
   //Write Value
   I2C_start();
   I2C_addr(0x80);
   I2C_write(0x00);
   I2C_write((register_return_value & ~(0x10)));
   I2C_stop();
   //Read current status
   I2C_start();
   I2C_addr(0x80);
   I2C_write(0x00);
   I2C_start();
   I2C_addr(0x80|1);
   register_return_value = I2C_read(0);
   I2C_stop();
   uint16_t pwm_value = 205;
 //
 I2C_start();
 I2C_addr(0x80);
 I2C_write(0x06);
 I2C_write(0x00);
 I2C_stop();
   //
 I2C_start();
 I2C_addr(0x80);
 I2C_write(0x07);
 I2C_write(0x00);
 I2C_stop();
 //
 I2C_start();
 I2C_addr(0x80);
 I2C_write(0x08);
 I2C_write(0xFF & pwm_value);
 I2C_stop();
 //
 I2C_start();
 I2C_addr(0x80);
 I2C_write(0x09);
 I2C_write(0x0F & (pwm_value << 8));
 I2C_stop();
}

• InBedded16: Please add your full code! I know the PCA very well and will try to help but seeing everything you're doing would be very helpful. Also some O-Scope captures would be great. When you write a value higher than 405, does it just max out at ~1ms no matter how high you go or is there other behavior?
• InBedded16: The pulse width decreasing suggests you're configuring the registers incorrectly. Seeing the register values for registers 0x01, 0x02, and 0x06-0x09 would be what to check.
• ulix: When I input increasing values, I get a pulse width that first increases and then decreases, but it never exceeds 1.2 ms. I will add the code below.
• ulix: @InBedded16 Registers 0x06 to 0x09 have the values I have configure them. 0x06=0x00; 0x07=0x00; 0x08=0xCD (for 205dec) and 0x09=0x00
• ulix: @InBedded16 sure you mean 0x01 and 0x02 not 0x00 and 0x01? either way: 0x01=0x04 and 0x02=0xe2; 0x00=0x01

Answer 1

I2C_write(0x0F & (pwm_value << 8));

This line is shifting pwm_value in the wrong direction, so it will always write 0 to the high byte of the register. It should be:

I2C_write(0x0F & (pwm_value >> 8));

• ulix: Thanks, just noticed that... should have noticed myself :)

Answer 2

The code writes PWM value incorrectly.

The register 8 is written correctly for the low byte, but register 9 gets always written as 0.

Basically after 255 the PWM value wraps back to 0 which would explain what you are experiencing.