Make the frequency selection more flexible on stm32f103

src/machine/machine_stm32f103.go

@@ -339,24 +339,23 @@ func (spi SPI) getBaudRate(config SPIConfig) uint32 {
 	var conf uint32
 
 	// set frequency dependent on PCLK2 prescaler (div 1)
-	switch config.Frequency {
-	case 125000:
+	switch {
+	case config.Frequency < 125000:
 		// Note: impossible to achieve lower frequency with current PCLK2!
 		conf |= stm32.SPI_CR1_BR_Div256
-	case 250000:
+	case config.Frequency < 250000:
 		conf |= stm32.SPI_CR1_BR_Div256
-	case 500000:
+	case config.Frequency < 500000:
 		conf |= stm32.SPI_CR1_BR_Div128
-	case 1000000:
+	case config.Frequency < 1000000:
 		conf |= stm32.SPI_CR1_BR_Div64
-	case 2000000:
+	case config.Frequency < 2000000:
 		conf |= stm32.SPI_CR1_BR_Div32
-	case 4000000:
+	case config.Frequency < 4000000:
 		conf |= stm32.SPI_CR1_BR_Div16
-	case 8000000:
-		conf |= stm32.SPI_CR1_BR_Div8
 	default:
-		conf |= stm32.SPI_CR1_BR_Div256
+		// When its bigger than Div16, just round to the maximum frequency.
+		conf |= stm32.SPI_CR1_BR_Div8
 	}
 	return conf << stm32.SPI_CR1_BR_Pos
 }