The DMA Wizard does not allow linking an 8-bit SAR ADC to an 8-bit VDAC.
It complains about incompatible burst sizes. The specific error is "The selected Source and Destination have incompatible Burst sizes. No valid size can be selected".
A minimal example project just containing the three relevant components added to the schematic is attached. This is enough to reproduce the error message. Also attached a screenshot of the error message.
I suspect this is due to the SAR ADC being permanently stuck in a 16-bit register configuration from the DMA's perspective, even if the component is configured with 8-bit resolution. This means that the DMA configuration assumes a minimum burst size of 2 bytes, even though a burst size of 1 byte should be fine.
In the DMA section of the SAR ADC datasheet, I see this comment about the ADC_SAR_WRK0_PTR source: "Receive a 2-byte result for a conversion with a result that always has 12-bit resolution." This supports the previous suspicion that the DMA burst size cannot shrink to match the configured 8-bit resolution.
The DMA Wizard scales the DeltaSigma ADC burst size correctly from 1 to 3 bytes as the resolution changes from 8-bit to 20-bit. The DeltaSigma datasheet also lists these burst sizes as valid lengths of ADC_DelSig_DEC_SAMP_PTR depending on resolution. I'm just surprised that the SAR ADC cannot scale burst width similarly.
Is this a bug, or just a limitation of the SAR ADC hardware?
Are there any workaround suggestions besides just using the DeltaSigma ADC instead of the SAR ADC?
I doubt it's possible to just point the DMA source address to ADC_SAR_WRK0_REG, which contains the relevant 8-bit value.
I'm thinking one option is to chain a pair of TDs that each read one byte, where the first TD passes the first byte along to the 8-bit VDAC, and the second TD just throws the other byte away.
I also found these two unresolved threads which encountered the same burst width issue.