altos/stm: Ensure SPI always sends 0xff during receive

SD cards require 0xff when fetching data

Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/src/stm/ao_spi_stm.c b/src/stm/ao_spi_stm.c
index 7b4af96443f28af949669e2d1cf18c2a49d2db49..56329c2446c43f6188a6f29dc3ae4d596a93c0df 100644 (file)
--- a/src/stm/ao_spi_stm.c
+++ b/src/stm/ao_spi_stm.c
@@ -160,6 +160,8 @@ ao_spi_recv(void *block, uint16_t len, uint8_t spi_index)
        uint8_t mosi_dma_index = ao_spi_stm_info[AO_SPI_INDEX(spi_index)].mosi_dma_index;
        uint8_t miso_dma_index = ao_spi_stm_info[AO_SPI_INDEX(spi_index)].miso_dma_index;
 
+       spi_dev_null = 0xff;
+
        /* Set up transmit DMA to make the SPI hardware actually run */
        ao_dma_set_transfer(mosi_dma_index,
                            &stm_spi->dr,