Data stored in the external memory can be seamlessly transferred to DAC using DMA and SPI. The
SPI can be configured to operate at an appropriate frequency (about 1 MHz) and in mode 0
(active high clock, active to idle clock edge, input data sampled at the end). The DMA
setup configurations are listed in Table 3-4.
Table 1. DMA Setup Configuration
(DMA3)
| DMA3 Setup |
| SSA |
Source Address |
&SPI1RXB |
| SSZ |
Source Size |
256 (1 page data from external memory) |
| SMR |
Source Memory Region |
SFR |
| SMODE |
Source Address Mode |
SPTR remains unchanged |
| DSA |
Destination Address |
&DAC1DATL |
| DSZ |
Destination Size |
1 |
| DMODE |
Destination Address Mode |
DPTR remains unchanged |
| SIRQEN |
Start Trigger Enable |
Yes |
| SIRQ |
Start Trigger |
TMR0 (sampling timer) |
| SSTP |
Source Counter Reload Stop |
Yes - SIRQEN is cleared when source counter
reloads |
| DSTP |
Destination Counter Reload Stop |
No - SIRQEN not cleared |
| AIRQEN |
Abort Trigger Enable |
No (stops when DCNT reloads) |
| AIRQ |
Abort Trigger |
N/A |
How It Works:
- 1.Before starting the DMA transfer,
ensure the microcontroller and external memory are ready to receive and transmit
data. To prepare the PIC18F57Q43 microcontroller
and SST26VF064B Flash module on Flash 2 Click Board™, the following steps are
required (refer to Hardware Setup Using Curiosity Nano Evaluation Kit for
hardware connection between the microcontroller and the flash module):
- 1.1.Pull the
chip
select line low and send the Read command
to the flash module.
- 1.2.Send the read address
(the address to read from the external memory) to the flash module.
- 1.3.Set SPI to operate in
Receive-only mode in PIC18F57Q43. This
allows the device to receive bursts of data without sending any data in
return.
- 2.Set SPI transmit counter to 256.
This immediately kicks off the SPI. Enable SIRQEN to kick off DMA at the next
timer overflow. The DMA and SPI transmit stop when SPI transmit counter depletes
to zero (and coincidentally SCNT reloads, clearing SIRQEN). The address pointer
in the external memory module is incremented automatically. Repeat this step to
read as many pages of stored audio data.
- 3.To close out the read operation,
restore SPI to Full-duplex mode and release the
chip select
line.
Important: In
Receive-only mode, SPI operations are suspended when the RXFIFO becomes full. In
this application, the SPI operation resumes only when RXFIFO is read by the DMA (at
the next TMR0 overflow event). This enables the playback and the data transfer rate
from external memory to match the sampling rate.