machine/rp2040: implement semi-random RNG based on ROSC based on pico-sdk

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/crypto/rand/rand_baremetal.go

@@ -1,5 +1,5 @@
-//go:build stm32 || (sam && atsamd51) || (sam && atsame5x)
-// +build stm32 sam,atsamd51 sam,atsame5x
+//go:build stm32 || (sam && atsamd51) || (sam && atsame5x) || rp2040
+// +build stm32 sam,atsamd51 sam,atsame5x rp2040
 
 package rand
 

src/machine/machine_rp2040_rng.go

@@ -0,0 +1,38 @@
+//go:build rp2040
+// +build rp2040
+
+// Implementation based on code located here:
+// https://github.com/raspberrypi/pico-sdk/blob/master/src/rp2_common/pico_lwip/random.c
+
+package machine
+
+import (
+	"device/rp"
+)
+
+const numberOfCycles = 32
+
+// GetRNG returns 32 bits of semi-random data based on ring oscillator.
+func GetRNG() (uint32, error) {
+	var val uint32
+	for i := 0; i < 4; i++ {
+		val = (val << 8) | uint32(roscRandByte())
+	}
+	return val, nil
+}
+
+var randomByte uint8
+
+func roscRandByte() uint8 {
+	var poly uint8
+	for i := 0; i < numberOfCycles; i++ {
+		if randomByte&0x80 != 0 {
+			poly = 0x35
+		} else {
+			poly = 0
+		}
+		randomByte = ((randomByte << 1) | uint8(rp.ROSC.GetRANDOMBIT()) ^ poly)
+		// TODO: delay a little because the random bit is a little slow
+	}
+	return randomByte
+}