samd51: use correct SPI frequency

The SPI frequency was rounded up, not rounded down. This meant that if
you wanted to configure 15MHz for example, it would pick the next
available frequency (24MHz). That's unsafe, the safe option is to round
down and the SPI support for most other chips also rounds down for this
reason.

In addition, I've improved SPI clock selection so that it will pick the
best clock of the two, widening the available frequencies. See the
comments in the patch for details.

src/machine/machine_atsamd51.go

@@ -1490,22 +1490,39 @@ func (spi SPI) Configure(config SPIConfig) error {
 		spi.Bus.CTRLA.ClearBits(sam.SERCOM_SPIM_CTRLA_CPOL)
 	}
 
-	// set clock
-	freqRef := uint32(0)
-	if config.Frequency > SERCOM_FREQ_REF/2 {
-		setSERCOMClockGenerator(spi.SERCOM, sam.GCLK_PCHCTRL_GEN_GCLK0)
-		freqRef = uint32(SERCOM_FREQ_REF_GCLK0)
-	} else {
-		setSERCOMClockGenerator(spi.SERCOM, sam.GCLK_PCHCTRL_GEN_GCLK1)
-		freqRef = uint32(SERCOM_FREQ_REF)
-	}
+	// Set the clock frequency.
+	// There are two clocks we can use GCLK0 (120MHz) and GCLK1 (48MHz).
+	// We can use any even divisor for these clock, which means:
+	//   - for GCLK0 we can make 60MHz, 30MHz, 20MHz, 15MHz, 12MHz, 10MHz, etc
+	//   - for GCLK1 we can make 24MHz, 12MHz, 8MHz, 6MHz, 4.8MHz, 4MHz, etc
+	// This means that by trying both clocks, we can have a wider selection of
+	// available SPI clock frequencies.
 
-	// Set synch speed for SPI
-	baudRate := freqRef / (2 * config.Frequency)
-	if baudRate > 0 {
-		baudRate--
+	// Calculate the baudrate if we would use GCLK1 (48MHz), and the resulting
+	// frequency. The baud rate is rounded up, so that the resulting frequency
+	// is rounded down from the maximum value (meaning it will always be smaller
+	// than or equal to config.Frequency).
+	baudRateGCLK1 := (SERCOM_FREQ_REF/2 + config.Frequency - 1) / config.Frequency
+	freqGCLK1 := SERCOM_FREQ_REF / 2 / baudRateGCLK1
+
+	// Same for GCLK0 (120MHz).
+	baudRateGCLK0 := (SERCOM_FREQ_REF_GCLK0/2 + config.Frequency - 1) / config.Frequency
+	freqGCLK0 := SERCOM_FREQ_REF_GCLK0 / 2 / baudRateGCLK0
+
+	// Pick the clock source that is the closest to the maximum baud rate.
+	// Note: there may be reasons to prefer the lower frequency clock (like
+	// power consumption). If that's the case, we might want to always use the
+	// 48MHz clock at low frequencies (below 4MHz or so).
+	if freqGCLK0 > freqGCLK1 && uint32(uint8(baudRateGCLK0-1))+1 == baudRateGCLK0 {
+		// Pick this 120MHz clock if it results in a better frequency after
+		// division, and the baudRate value fits in the BAUD register.
+		setSERCOMClockGenerator(spi.SERCOM, sam.GCLK_PCHCTRL_GEN_GCLK0)
+		spi.Bus.BAUD.Set(uint8(baudRateGCLK0 - 1))
+	} else {
+		// Use the 48MHz clock in other cases.
+		setSERCOMClockGenerator(spi.SERCOM, sam.GCLK_PCHCTRL_GEN_GCLK1)
+		spi.Bus.BAUD.Set(uint8(baudRateGCLK1 - 1))
 	}
-	spi.Bus.BAUD.Set(uint8(baudRate))
 
 	// Enable SPI port.
 	spi.Bus.CTRLA.SetBits(sam.SERCOM_SPIM_CTRLA_ENABLE)