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SPI DAC Demo

This example shows how to set up the SPI port with a timer, circular DMA and IRQs to do continuous audio output through an inexpensive I2S DAC.

Scope image of sine and sawtooth waves

Theory

The CH32V003 does not have an I2S port which is usually required for driving audio DACs, but the inexpensive PT8211 stereo audio DAC is very forgiving of the signal format used to drive it so we can approximate an I2S signal using one of the CH32V003 on-chip timers to generate the frame sync signal. The SPI port then provides the bit clock and serial data.

Timer setup

TIM1 on the CH32V003 is set up to generate a 48kHz square wave which is output on GPIO pin PC4 and serves as the frame sync or WS. The timer is configured to run in center-aligned mode 3 which generates DMA requests on both rising and falling edges. Channel 4 is configured as the output and the threshold is set to 50% and the overall period is set to 48kHz.

SPI setup

SPI1 is configured for 16-bit TX with a bit clock of 48MHz/8 (3MHz) which is fast enough to clock out 16 bits of data between the edges of the frame sync signal. Transmit data arrives via DMA, but the SPI port does not control DMA - that is triggered from the timer above.

DMA setup

DMA1 channel 4 is used because that channel is connected to TIM1 Chl 4 output and is set up in circular mode with both Half-transfer and Transfer-Complete interrupts. It continuously pulls data out of a 32-word buffer and sends it to the SPI port when the timer fires.

Interrupts

The DMA TC and HT IRQs trigger execution of a buffer fill routine which simply indexes through a sinewave table at variable rates using fixed-point math to fill the buffer as requested. This process uses up only about 4% of the available CPU cycles so there's plenty leftover for foreground processing or more complex waveform calculations like interpolation or synthesis.

Use

Connect a PT8211 DAC as follows:

  • DAC pin 1 (BCK) - MCU pin 15 (PC5/SCK)
  • DAC pin 2 (WS) - MCU pin 14 (PC4/T1CH4)
  • DAC pin 3 (DIN) - MCU pin 16 {PC6/MOSI)
  • DAC pin 4 (GND) - ground
  • DAC pin 5 (VCC) - 3.3V or 5V supply
  • DAC pin 6 (LCH) - left channel output
  • DAC pin 8 (RCH) - right channel output

Schematic of SPI DAC hookup

Connect an oscilloscope to the left and right channel outputs and observe a sine waveform at 187Hz on the right channel output and a sawtooth wave at 47Hz on the left channel output.