stm32: Wait for transmission to complete before returning from spi_transfer()

It's possible for the SCLK pin to still be updating even after the
last byte of data has been read from the receive pin.  (In particular
in spi mode 0 and 1.)  Exiting early from spi_transfer() in this case
could result in the CS pin being raised before the final updates to
SCLK pin.

Add an additional wait at the end of spi_transfer() to avoid this
issue.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/stm32/spi.c b/src/stm32/spi.c
index b78196d8..f1ba33cf 100644
--- a/src/stm32/spi.c
+++ b/src/stm32/spi.c
@@ -109,12 +109,15 @@ spi_transfer(struct spi_config config, uint8_t receive_data,
 {
     SPI_TypeDef *spi = config.spi;
     while (len--) {
-        writeb((void *)&spi->DR, *data);
+        writeb((void*)&spi->DR, *data);
         while (!(spi->SR & SPI_SR_RXNE))
             ;
-        uint8_t rdata = readb((void *)&spi->DR);
+        uint8_t rdata = readb((void*)&spi->DR);
         if (receive_data)
             *data = rdata;
         data++;
     }
+    // Wait for any remaining SCLK updates before returning
+    while ((spi->SR & (SPI_SR_TXE|SPI_SR_BSY)) != SPI_SR_TXE)
+        ;
 }