STM32 DAC PA4/PA5

Posted on November 12, 2015 at 20:21

I am generating music using STM32L152RC.

I have DAC configured the same for both channels DAC_OUT1/2 using PA4/PA5.

RCC->APB1ENR |= RCC_APB1ENR_DACEN; 
// Enable DAC
RCC->AHBENR |= RCC_AHBENR_GPIOAEN; 
// enable GPIOA
// PA4/5 Analog
GPIOA->MODER |= 0x00000F00;
// Enable DAC channel1 output buffer
DAC->CR &= ~(DAC_CR_BOFF1 | DAC_CR_BOFF2); 
// Select TIM4 TRGO as trigger
DAC->CR |= (DAC_CR_TEN1 | DAC_CR_TEN2); 
// Enable DAC channel 1 & 2 Trigger 
DAC->CR &= ~(DAC_CR_TSEL1 | DAC_CR_TSEL2); 
DAC->CR |= (DAC_CR_TSEL1_2 | DAC_CR_TSEL1_0); 
// 101 = TIM4_TRGO
DAC->CR |= (DAC_CR_TSEL2_2 | DAC_CR_TSEL2_0); 
// 101 = TIM4_TRGO 
// Enable DAC
DAC->CR |= (DAC_CR_EN1 | DAC_CR_EN2); 
// DAC channel 1 & 2 enable

I write to the DAC thus:

static
uint32_t ch2out, ch1out;
............
ch2out = (y << 16);
ch1out = (y);
DAC->DHR12RD = ch2out | ch1out;
............

So here's the problem. When I connect PA5 to my speaker setup (an LM386 opamp), it sounds exactly as expected. But when I disconnect PA5 and connect PA4 to the same exact setup, changing nothing else, the output is inaudible. From the code, you can see that PA4/PA5 , DAC_OUT1/DAC_OUT2 are configured exactly the same, and exactly the same signal is being written to each channel. When I disconnect both PA4/5 from the circuit and used an oscilloscope with my code running, they both show exactly the same output. Yet somehow connecting PA4 to an actual circuit is not working. Also, I tried the program on a second board, and got the same results. I'm not really sure what to make of it. How are PA4 and PA5 different? What am I missing?