machine/samd51: implement TRNG for randomness

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

src/crypto/rand/rand_baremetal.go

@@ -1,5 +1,5 @@
-//go:build stm32wlx
-// +build stm32wlx
+//go:build stm32wlx || (sam && atsamd51) || (sam && atsame5x)
+// +build stm32wlx sam,atsamd51 sam,atsame5x
 
 package rand
 

src/machine/machine_atsamd51.go

@@ -1,3 +1,4 @@
+//go:build (sam && atsamd51) || (sam && atsame5x)
 // +build sam,atsamd51 sam,atsame5x
 
 // Peripheral abstraction layer for the atsamd51.
@@ -2733,3 +2734,20 @@ func syncDAC() {
 	for sam.DAC.SYNCBUSY.HasBits(sam.DAC_SYNCBUSY_DATA0) {
 	}
 }
+
+var rngInitDone = false
+
+// GetRNG returns 32 bits of cryptographically secure random data
+func GetRNG() (uint32, error) {
+	if !rngInitDone {
+		// Turn on clock for TRNG
+		sam.MCLK.APBCMASK.SetBits(sam.MCLK_APBCMASK_TRNG_)
+
+		// enable
+		sam.TRNG.CTRLA.Set(sam.TRNG_CTRLA_ENABLE)
+
+		rngInitDone = true
+	}
+	ret := sam.TRNG.DATA.Get()
+	return ret, nil
+}