LPC15xx SPI DMA demo code

Overview

This code demonstrates how to use the DMA controller with SPI on the LPC15xx processor (specifically the LPC1549). While it's targeted for the OM13056 Xpresso LPC1549 board, it only uses the LEDs. The SPI port is routed to the J1 connector, using the same pins the Arduino would.

The demo implements a circular buffer that DMA requests are stored into. This allows code to queue up several requests without waiting for the first one to complete. If a request is made and the buffer is full, then a wait loop is run until an entry is free (this uses the FreeRTOS vTaskDelay() call, which will yield to other tasks).

As each DMA requests completes, if there are additional entries in the circular buffer, the DMA interrupt handler will start the next entry.

If the length of the DMA request is <= 1024 words (where a word is the size of each entry, which can be 1, 2, or 4 bytes), the dmaSPI() function can be used. If the request is > 1024 words, then the dmaSPIEx() function can be used, which will fragment the requests into blocks of 1024 words.

The demo will send 26 strings of 32 characters to the SPI0 port, delay 1 second, and then repeat forever. The red LED (PIO0_25) will blink at a rate of 4Hz, while the green LED (PIO0_3) will blink at a rate of 8Hz. The blue LED (PIO1_1) is not used.

NOTE: This code only transmits. In the SPI0 configuration you'll find SPI_TXCTL_RXIGNORE is set. If you wish to use DMA to both transmit and receive at the same time, you'll need to setup a DMA receive transfer. Basically, anywhere you find a reference to DMAREQ_SPI0_TX, you'll need corresponding code for DMAREQ_SPI0_RX.

I found that if you don't set the SPI_TXCTL_RXIGNORE option, the DMA handler would never fire. If you send a single character with Chip_SPI_WriteFrames_Blocking() then the DMA handler would fire. I didn't delve any deeper in to it, but I suspect the SPI receive overflow error bit gets set and causes something to happen. If you figure that out, post it a PR or an issue and I'll integrate it.

Build environment

Uses arm-gcc and make. The LPC15xx v2.20 libraries are included. There are two warnings emitted when ring_buffer.c is compiled. I added GCC attributes to suppress unused variable warnings in the library (who writes code that's allowed to generate warnings? No excuse!)