stm32wl: STM32WL TRNG implementation in crypto/rand

src/machine/machine.go

@@ -3,6 +3,7 @@ package machine
 import "errors"
 
 var (
+	ErrTimeoutRNG         = errors.New("machine: RNG Timeout")
 	ErrInvalidInputPin    = errors.New("machine: invalid input pin")
 	ErrInvalidOutputPin   = errors.New("machine: invalid output pin")
 	ErrInvalidClockPin    = errors.New("machine: invalid clock pin")

src/machine/machine_stm32wle5.go

@@ -13,6 +13,8 @@ import (
 	"unsafe"
 )
 
+var rngInitDone = false
+
 const (
 	AF0_SYSTEM             = 0
 	AF1_TIM1_2_LPTIM1      = 1
@@ -30,9 +32,10 @@ const (
 )
 
 const (
-	SYSCLK        = 48e6
-	APB1_TIM_FREQ = SYSCLK
-	APB2_TIM_FREQ = SYSCLK
+	SYSCLK               = 48e6
+	APB1_TIM_FREQ        = SYSCLK
+	APB2_TIM_FREQ        = SYSCLK
+	RNG_MAX_READ_RETRIES = 1000
 )
 
 func CPUFrequency() uint32 {
@@ -396,6 +399,27 @@ func (t *TIM) enableMainOutput() {
 	t.Device.BDTR.SetBits(stm32.TIM_BDTR_MOE)
 }
 
+// GetRNG returns 32 bits of cryptographically secure random data
+func GetRNG() (uint32, error) {
+
+	if !rngInitDone {
+		stm32.RCC.AHB3ENR.SetBits(stm32.RCC_AHB3ENR_RNGEN)
+		stm32.RNG.CR.SetBits(stm32.RNG_CR_RNGEN)
+		rngInitDone = true
+	}
+	cnt := RNG_MAX_READ_RETRIES
+	for cnt > 0 && !stm32.RNG.SR.HasBits(stm32.RNG_SR_DRDY) {
+		cnt--
+	}
+	if cnt == 0 {
+		return 0, ErrTimeoutRNG
+	}
+	ret := stm32.RNG.DR.Get()
+	return ret, nil
+}
+
+//----------
+
 type arrtype = uint32
 type arrRegType = volatile.Register32