GPIO toggle, uneven high and low.

[arunashok]

Posted on December 09, 2011 at 14:39

Hi,

 I want to measure the clock of my stm32f225rg chip. I am doing it by toggling the bit of gpio port B and checking using scope.  Rather than using the routines in the standard peripheral library, I am programming the registers directly.  The routine for toggling is below. while (1)       {         GPIOB->BSRRL = GPIO_Pin_15;     // Set         GPIOB->BSRRH = GPIO_Pin_15;      // ReSet         GPIOB->BSRRL = GPIO_Pin_15;     // Set         GPIOB->BSRRH = GPIO_Pin_15;      // ReSet         GPIOB->BSRRL = GPIO_Pin_15;     // Set         GPIOB->BSRRH = GPIO_Pin_15;      // ReSet         GPIOB->BSRRL = GPIO_Pin_15;     // Set         GPIOB->BSRRH = GPIO_Pin_15;      // ReSet         GPIOB->BSRRL = GPIO_Pin_15;     // Set         GPIOB->BSRRH = GPIO_Pin_15;      // ReSet         GPIOB->BSRRL = GPIO_Pin_15;     // Set         Delay(200);         GPIOB->BSRRH = GPIO_Pin_15;     // ReSet         Delay(200);       } The toggling can be observed in the scope, but I find the on and off times different. Though the number of instructions for the bitset and bitreset are 4 it seems bitset is using 5 clock cycles and bitreset 4. Any help  would be appreciated.

[Tesla DeLorean]

Posted on December 09, 2011 at 16:11

The GPIO unit is on the APB bus, and behind a write buffer, figure at least 4 cycles. Toggling the pins in this fashion is awfully inefficient. Performance might be different if you use 32-bit writes, and code this in assembler.

Use a TIMer to generate signals with predictable duty cycles. Use the MCO pin to measure CPU frequency. Run code from RAM to avoid unpredictable behaviour of the flash controller/cache.