How to efficiently use GPDMA to generate audio waveform on DAC output pin

[lpcware]

Content originally posted in LPCWare by giusloq on Fri May 08 03:39:23 MST 2015
I need to play some simple audio signals on DAC output pin and I'd like to use GPDMA so I can save CPU cycles.
I don't need hi-fi, so I'm going to store audio signals as 8-bits const unsigned char arrays.

The single 8-bit sample should be written to DACR register, but it must be manipulated. I don't need to save power (BIAS flag could be set to 0), so I have to only shift the 8-bit sample value by 8 bits:

  const unsigned char samples[] = { ... };
  ...
  DACR = samples[idx] << 8;

In other words, I have to write the 8-bits sample to address 0x4008 C001 and not the base DACR register 0x4008 C000.

Is it possible with GPDMA and LPCOpen libraries?

[lpcware]

Content originally posted in LPCWare by wmues on Sat May 09 09:08:44 MST 2015
Make a copy of the DMA setup code, and change the copy to do what you want.

[lpcware]

Content originally posted in LPCWare by giusloq on Sat May 09 06:23:16 MST 2015
I started from the periph_dac example project that comes with LPCOpen:

Chip_GPDMA_Transfer(LPC_GPDMA, dmaChannelNum,
  (uint32_t) &DMAbuffer,
  GPDMA_CONN_DAC,
  GPDMA_TRANSFERTYPE_M2P_CONTROLLER_DMA,
  1);

Chip_GPDMA_Transfer() calls Chip_GPDMA_InitChannelCfg() that is:

volatile static const void *GPDMA_LUTPerAddr[] = {
0,/* Reserved */
[...]
(&LPC_DAC->CR),/* DAC */
[...]
};
int Chip_GPDMA_InitChannelCfg(LPC_GPDMA_T *pGPDMA,
  GPDMA_CH_CFG_T *GPDMACfg,
  uint8_t  ChannelNum,
  uint32_t src,
  uint32_t dst,
  uint32_t Size,
  GPDMA_FLOW_CONTROL_T TransferType)
{
int rval = -1;
GPDMACfg->ChannelNum = ChannelNum;
GPDMACfg->TransferType = TransferType;
GPDMACfg->TransferSize = Size;

switch (TransferType) {
[...]
case GPDMA_TRANSFERTYPE_M2P_CONTROLLER_DMA:
case GPDMA_TRANSFERTYPE_M2P_CONTROLLER_PERIPHERAL:
GPDMACfg->SrcAddr = (uint32_t) src;
rval = 1;
GPDMACfg->DstAddr = (uint32_t) GPDMA_LUTPerAddr[dst];
break;
[...]
}

With this tranfer type (GPDMA_TRANSFERTYPE_M2P_CONTROLLER_DMA) the destination address is the full DAC value register, so I can't change it.

The only way to set an arbitrary destination address is to use GPDMA_TRANSFERTYPE_M2M_CONTROLLER_DMA transfer type, but in this case:

int Chip_GPDMA_InitChannelCfg(LPC_GPDMA_T *pGPDMA,
  GPDMA_CH_CFG_T *GPDMACfg,
  uint8_t  ChannelNum,
  uint32_t src,
  uint32_t dst,
  uint32_t Size,
  GPDMA_FLOW_CONTROL_T TransferType)
{
[...]
switch (TransferType) {
case GPDMA_TRANSFERTYPE_M2M_CONTROLLER_DMA:
GPDMACfg->SrcAddr = (uint32_t) src;
GPDMACfg->DstAddr = (uint32_t) dst;
rval = 3;
GPDMACfg->TransferWidth = GPDMA_WIDTH_WORD;
GPDMACfg->TransferSize = Size / 4;
break;
[...]
}

The transfer width is fixed to GPDMA_WIDTH_WORD, that is 4 bytes.

Should I have to change LPCOpen chip-level functions?

[lpcware]

Content originally posted in LPCWare by wmues on Fri May 08 07:31:27 MST 2015
The GPDMA can write bytes to every address. So yes, this will be possible.