Merge pull request #13537 from jue89/feature/sam0-spi-frequency

cpu/sam0_common: mitigate rounding errors of SPI baud rate calculation
RIOT-OS · Mar 4, 2020 · 681678e · 681678e
2 parents 0c2c5f2 + 8fbfb13
commit 681678e
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Showing 2 changed files with 6 additions and 3 deletions.
diff --git a/boards/samr30-xpro/include/board.h b/boards/samr30-xpro/include/board.h
@@ -36,7 +36,7 @@ extern "C" {
 #define AT86RF2XX_PARAM_INT         GPIO_PIN(PB, 0)
 #define AT86RF2XX_PARAM_SLEEP       GPIO_PIN(PA, 20)
 #define AT86RF2XX_PARAM_RESET       GPIO_PIN(PB, 15)
-#define AT86RF2XX_PARAM_SPI_CLK     SPI_CLK_1MHZ
+#define AT86RF2XX_PARAM_SPI_CLK     SPI_CLK_5MHZ
 /** @}*/
 
 /**

diff --git a/cpu/sam0_common/periph/spi.c b/cpu/sam0_common/periph/spi.c
@@ -103,8 +103,11 @@ int spi_acquire(spi_t bus, spi_cs_t cs, spi_mode_t mode, spi_clk_t clk)
 
     /* configure bus clock, in synchronous mode its calculated from
      * BAUD.reg = (f_ref / (2 * f_bus) - 1)
-     * with f_ref := CLOCK_CORECLOCK as defined by the board */
-    const uint8_t baud = (sam0_gclk_freq(spi_config[bus].gclk_src) / (2 * clk) - 1);
+     * with f_ref := CLOCK_CORECLOCK as defined by the board
+     * to mitigate the rounding error due to integer arithmetic, the
+     * equation is modified to
+     * BAUD.reg = ((f_ref + f_bus) / (2 * f_bus) - 1) */
+    const uint8_t baud = ((sam0_gclk_freq(spi_config[bus].gclk_src) + clk) / (2 * clk) - 1);
 
     /* configure device to be master and set mode and pads,
      *