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cpu/sam0_common: mitigate rounding errors of baud rate calculation

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Instead of always rounding up, the driver now rounds to the nearest integer. This reduces the absolut rounding error when setting SPI baud rates.
This commit is contained in:
Juergen Fitschen 2020-03-03 12:24:42 +01:00
parent d13d1aed48
commit 0032d35fe8
1 changed files with 5 additions and 2 deletions
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7
cpu/sam0_common/periph/spi.c
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@ -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,
*