begin adding sd.Card refactor

examples/sd/main.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"fmt"
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/sd"
+)
+
+const (
+	SPI_RX_PIN  = machine.GP16
+	SPI_TX_PIN  = machine.GP19
+	SPI_SCK_PIN = machine.GP18
+	SPI_CS_PIN  = machine.GP15
+)
+
+var (
+	spibus = machine.SPI0
+)
+
+func main() {
+	time.Sleep(time.Second)
+	SPI_CS_PIN.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	err := spibus.Configure(machine.SPIConfig{
+		Frequency: 250000,
+		Mode:      0,
+		SCK:       SPI_SCK_PIN,
+		SDO:       SPI_TX_PIN,
+		SDI:       SPI_RX_PIN,
+	})
+	if err != nil {
+		panic(err.Error())
+	}
+	sdcard := sd.NewCard(spibus, SPI_CS_PIN.Set)
+
+	err = sdcard.Init()
+	if err != nil {
+		panic(err.Error())
+	}
+	csd := sdcard.CSD()
+	fmt.Printf("cid=%+v\ncsd=%+v\ncsdfmt=\n%s\ndone!", sdcard.CID(), csd, csd.String())
+
+	var buf [512]byte
+	for i := 1; i < 11; i += 1 {
+		time.Sleep(time.Millisecond)
+		err = sdcard.ReadBlock(uint32(i), buf[:])
+		if err != nil {
+			println("err reading block", i, ":", err.Error())
+			continue
+		}
+		fmt.Printf("block %d crc=%#x:\n\t%#x\n", i, sdcard.LastReadCRC(), buf[:])
+	}
+}