machine/nrf52840: implement RNG peripheral operation

src/crypto/rand/rand_baremetal.go

@@ -1,5 +1,5 @@
-//go:build stm32 || (sam && atsamd51) || (sam && atsame5x) || rp2040
-// +build stm32 sam,atsamd51 sam,atsame5x rp2040
+//go:build nrf52840 || stm32 || (sam && atsamd51) || (sam && atsame5x) || rp2040
+// +build nrf52840 stm32 sam,atsamd51 sam,atsame5x rp2040
 
 package rand
 

src/machine/machine_nrf52840_rng.go

@@ -0,0 +1,60 @@
+//go:build nrf52840
+// +build nrf52840
+
+package machine
+
+import (
+	"device/nrf"
+)
+
+// Implementation based on Nordic Semiconductor's nRF52840 documentation version 1.7 found here:
+// https://infocenter.nordicsemi.com/pdf/nRF52840_PS_v1.7.pdf
+
+// SetRNGBiasCorrection configures the RNG peripheral's bias correction mechanism. Note that when
+// bias correction is enabled, the peripheral is slower to produce random values.
+func SetRNGBiasCorrection(enabled bool) {
+	var val uint32
+	if enabled {
+		val = nrf.RNG_CONFIG_DERCEN_Enabled
+	}
+	nrf.RNG.SetCONFIG_DERCEN(val)
+}
+
+// RNGBiasCorrectionEnabled determines whether the RNG peripheral's bias correction mechanism is
+// enabled or not.
+func RNGBiasCorrectionEnabled() bool {
+	return nrf.RNG.GetCONFIG_DERCEN() == nrf.RNG_CONFIG_DERCEN_Enabled
+}
+
+// StartRNG starts the RNG peripheral core. This is automatically called by GetRNG, but can be
+// manually called for interacting with the RNG peripheral directly.
+func StartRNG() {
+	nrf.RNG.SetTASKS_START(nrf.RNG_TASKS_START_TASKS_START_Trigger)
+}
+
+// StopRNG stops the RNG peripheral core. This is not called automatically. It may make sense to
+// manually disable RNG peripheral for power conservation.
+func StopRNG() {
+	nrf.RNG.SetTASKS_STOP(nrf.RNG_TASKS_STOP_TASKS_STOP_Trigger)
+}
+
+// GetRNG returns 32 bits of non-deterministic random data based on internal thermal noise.
+// According to Nordic's documentation, the random output is suitable for cryptographic purposes.
+func GetRNG() (ret uint32, err error) {
+	// There's no apparent way to check the status of the RNG peripheral's task, so simply start it
+	// to avoid deadlocking while waiting for output.
+	StartRNG()
+
+	// The RNG returns one byte at a time, so stack up four bytes into a single uint32 for return.
+	for i := 0; i < 4; i++ {
+		// Wait for data to be ready.
+		for nrf.RNG.GetEVENTS_VALRDY() == nrf.RNG_EVENTS_VALRDY_EVENTS_VALRDY_NotGenerated {
+		}
+		// Append random byte to output.
+		ret = (ret << 8) ^ nrf.RNG.GetVALUE()
+		// Unset the EVENTS_VALRDY register to avoid reading the same random output twice.
+		nrf.RNG.SetEVENTS_VALRDY(nrf.RNG_EVENTS_VALRDY_EVENTS_VALRDY_NotGenerated)
+	}
+
+	return ret, nil
+}