DUE SD card access using DMA

sage123 · September 14, 2018, 4:23am

I'm looking for a way to permanently read data from an SD card while keeping the workload of the CPU as low as possible.
I imagine that I just give a pointer to a buffer to a DMA based SD.read routine which fills the buffer via DMA without loading the CPU and just issues an interrupt when the buffer is full (like the DAC of the DUE when operated via DMA)
I thought this could be done with the SdFat Library as it is apparently able to use DMA. However it seems to use it only for speeding up the read process, but not for filling a buffer in the background.

Does anyone know how to do something like this?

Thank you in advance for your help.

ard_newbie · September 14, 2018, 6:55am

There is an AHB DMA for the SPI peripheral (Sam3x datasheet, page 339). You first need to initialize the SPI reading of the SD card with the correct pace, and then initialize a single AHB DMA transfer, or multiple AHB DMA transfers with a linked list item (LLI) from the SPI peripheral to the memory and this will be done with the correct pace.

github.com

openbci-archive/OpenBCI_8bit/blob/master/OpenBCI_8bit_SDfat_Library/SdFat/SdSpiSAM3X.cpp

/* Arduino SdSpi Library
 * Copyright (C) 2013 by William Greiman
 *
 * This file is part of the Arduino SdSpi Library
 *
 * This Library is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This Library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with the Arduino SdSpi Library.  If not, see
 * <http://www.gnu.org/licenses/>.
 */
#include <SdSpi.h>

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github.com

anydream/TurboSPI/blob/master/TurboSPI_SAM3X.cpp


#include "TurboSPI.h"

#if defined(__SAM3X8E__) || defined(__SAM3X8H__)
/** Use SAM3X DMAC if nonzero */
#define USE_SAM3X_DMAC 1
/** Use extra Bus Matrix arbitration fix if nonzero */
#define USE_SAM3X_BUS_MATRIX_FIX 0
/** Time in ms for DMA receive timeout */
#define SAM3X_DMA_TIMEOUT 100
/** chip select register number */
#define SPI_CHIP_SEL 3
/** DMAC receive channel */
#define SPI_DMAC_RX_CH  1
/** DMAC transmit channel */
#define SPI_DMAC_TX_CH  0
/** DMAC Channel HW Interface Number for SPI TX. */
#define SPI_TX_IDX  1
/** DMAC Channel HW Interface Number for SPI RX. */
#define SPI_RX_IDX  2

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DUE SD card access using DMA

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