Hey Guys,
So I just finished up a program that pushes a sine wave to the DAC on the photon using DMA. However, I have a strange issue. It would appear that the max voltage (4095) corresponds to ~180 mV when output by the DAC with DMA vs. 4095 corresponding to 3.3 V with standard analog writes. I measured the waveform with a scope and I can ensure that it is being is generated properly (with the correct number of divisions and frequency), except for the shift in the upper reference voltage. (Albeit there is some clipping due to the 50 mV lower limit on output writes, but this is an easy fix). I was wondering if I did something strange in my code, or if there is an option to set a external or internal reference for the DAC that I didn’t see.
void dacDMAConfig (uint16_t bufferSize, uint16_t *buffer) {
Serial.printlnf("Configuring DAC DMA");
//Enable dma clk.
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA1, ENABLE);
/* DMA1 Stream5 channel7 configuration */
DMA_DeInit(DMA1_Stream5);
DMA_InitTypeDef DMA_InitStructure;
DMA_StructInit(&DMA_InitStructure);
DMA_InitStructure.DMA_BufferSize = bufferSize;
DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t) &_DAC->DHR12L1;
DMA_InitStructure.DMA_Memory0BaseAddr = (uint32_t)buffer;
DMA_InitStructure.DMA_DIR = DMA_DIR_MemoryToPeripheral;
DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;
DMA_InitStructure.DMA_PeripheralDataSize = DMA_PeripheralDataSize_HalfWord;
DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_HalfWord;
DMA_InitStructure.DMA_Mode = DMA_Mode_Circular;
DMA_InitStructure.DMA_Priority = DMA_Priority_High;
DMA_InitStructure.DMA_Channel = DMA_Channel_7; // Stream5 Channel7 = DAC1
DMA_Init(DMA1_Stream5, &DMA_InitStructure);
/* Enable DMA1 Stream5 */
DMA_Cmd(DMA1_Stream5, ENABLE);
}
void dacConfig (uint16_t bufferSize, uint16_t *buffer) {
Serial.printlnf("Configuring DAC");
dacDMAConfig(bufferSize, buffer);
// DAC clk enable.
RCC_APB1PeriphClockCmd(RCC_APB1Periph_DAC, ENABLE);
/* DAC channel1 Configuration */
DAC_DeInit();
DAC_InitTypeDef DAC_InitStructure;
DAC_StructInit(&DAC_InitStructure);
DAC_InitStructure.DAC_Trigger = DAC_Trigger_T5_TRGO;
DAC_InitStructure.DAC_WaveGeneration = DAC_WaveGeneration_None;
DAC_InitStructure.DAC_OutputBuffer = DAC_OutputBuffer_Enable;
DAC_Init(DAC_Channel_1, &DAC_InitStructure);
}
void dacStart () {
/* Enable DAC Channel1 */
DAC_Cmd(DAC_Channel_1, ENABLE);
/* Enable DMA for DAC Channel1 */
DAC_DMACmd(DAC_Channel_1, ENABLE);
Serial.printlnf("Starting DAC");
}
Anyway, this one has me stumped, so any help would be appreciated.
Thanks!
It was the first thing I checked… I have absolutely no idea why is is happening. It is the same result on the other DAC. Vref+ cannot be lower than 1.8V so that suggests that there is some kind of scaling unit being enabled. Unfortunately, I cannot seem to find any mention of such a unit in the reference manual. I checked to see if the internal reference for the ADC was being enabled, but no luck there either. It works 100% as expected when using analogWrite(), so I am not sure what could possibly be causing this drastic change in the output voltage.