add config baud increase docs

examples/sd/main.go

@@ -17,18 +17,19 @@ const (
 
 var (
 	spibus = machine.SPI0
-)
-
-func main() {
-	time.Sleep(time.Second)
-	SPI_CS_PIN.Configure(machine.PinConfig{Mode: machine.PinOutput})
-	err := spibus.Configure(machine.SPIConfig{
+	spicfg = machine.SPIConfig{
 		Frequency: 250000,
 		Mode:      0,
 		SCK:       SPI_SCK_PIN,
 		SDO:       SPI_TX_PIN,
 		SDI:       SPI_RX_PIN,
-	})
+	}
+)
+
+func main() {
+	time.Sleep(time.Second)
+	SPI_CS_PIN.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	err := spibus.Configure(spicfg)
 	if err != nil {
 		panic(err.Error())
 	}
@@ -38,9 +39,14 @@ func main() {
 	if err != nil {
 		panic("sd card init:" + err.Error())
 	}
+	// After initialization it's safe to increase SPI clock speed.
+	csd := sdcard.CSD()
+	kbps := csd.TransferSpeed().RateKilobits()
+	spicfg.Frequency = uint32(kbps * 1000)
+	err = spibus.Configure(spicfg)
+
 	cid := sdcard.CID()
 	pname := cid.ProductName()
-	csd := sdcard.CSD()
 	if !cid.IsValid() {
 		copy := cid.RawCopy()
 		println("CID not valid: theirCRC=", cid.CRC7(), "ourCRC=", sd.CRC7(copy[:15]))

sd/card.go

@@ -58,6 +58,8 @@ func (c *SPICard) csEnable(b bool) { c.cs(!b) }
 func (c *SPICard) LastReadCRC() uint16 { return c.lastCRC }
 func (c *SPICard) LastR1() r1          { return c.lastr1 }
 
+// Init initializes the SD card. This routine should be performed with a SPI clock
+// speed of around 100..400kHz. One may increase the clock speed after initialization.
 func (d *SPICard) Init() error {
 	dummy := d.buf[:]
 	for i := range dummy {

sd/definitions.go

@@ -447,7 +447,7 @@ var log10table = [...]int64{
 	1000000,
 }
 
-// RateMegabits returns the transfer rate in megabits per second.
+// RateMegabits returns the transfer rate in kilobits per second.
 func (t TransferSpeed) RateKilobits() int64 {
 	return 100 * log10table[t&0b111]
 }