samd/moduos: Add uos.urandom() using the phase-jitter rng.

This RNG passes many of the Diehard tests and also the AIS31 test suite.
The RNG is quite slow, delivering 200bytes/s.

Tested on boards with and without a crystal.

3 files changed, 22 insertions, 12 deletions

diff --git a/ports/samd/mcu/samd21/mpconfigmcu.h b/ports/samd/mcu/samd21/mpconfigmcu.h
index 150a378a3..db4e58753 100644
--- a/ports/samd/mcu/samd21/mpconfigmcu.h
+++ b/ports/samd/mcu/samd21/mpconfigmcu.h
@@ -22,8 +22,8 @@
 #define MICROPY_PY_CMATH                (0)
 #endif
 
-#define MICROPY_PY_URANDOM_SEED_INIT_FUNC (trng_random_u32())
-unsigned long trng_random_u32(void);
+#define MICROPY_PY_URANDOM_SEED_INIT_FUNC (trng_random_u32(300))
+unsigned long trng_random_u32(int delay);
 
 #define VFS_BLOCK_SIZE_BYTES            (1536) // 24x 64B flash pages;
 
@@ -36,6 +36,7 @@ unsigned long trng_random_u32(void);
 #define MICROPY_PY_MACHINE_RTC          (1)
 #endif
 #endif
+#define MICROPY_PY_UOS_URANDOM          (1)
 
 #ifndef MICROPY_PY_MACHINE_PIN_BOARD_CPU
 #define MICROPY_PY_MACHINE_PIN_BOARD_CPU (1)
diff --git a/ports/samd/moduos.c b/ports/samd/moduos.c
index e0237fd88..3404919d2 100644
--- a/ports/samd/moduos.c
+++ b/ports/samd/moduos.c
@@ -52,6 +52,23 @@ uint32_t trng_random_u32(void) {
     return REG_TRNG_DATA;
 }
 
+#define TRNG_RANDOM_U32 trng_random_u32()
+#endif // defined(MCU_SAMD51)
+
+#if defined(MCU_SAMD21)
+extern volatile uint32_t rng_state;
+
+uint32_t trng_random_u32(int delay) {
+    mp_hal_delay_ms(delay); // wait a few cycles
+    uint32_t upper = rng_state;
+    mp_hal_delay_ms(delay); // wait again a few cycles
+    return (upper & 0xffff0000) | (rng_state >> 16);
+}
+
+#define trng_start()
+#define TRNG_RANDOM_U32 trng_random_u32(10)
+#endif
+
 #if MICROPY_PY_UOS_URANDOM
 STATIC mp_obj_t mp_uos_urandom(mp_obj_t num) {
     mp_int_t n = mp_obj_get_int(num);
@@ -62,7 +79,7 @@ STATIC mp_obj_t mp_uos_urandom(mp_obj_t num) {
     trng_start();
     for (int i = 0; i < n; i++) {
         if ((i % 4) == 0) {
-            rngval = trng_random_u32();
+            rngval = TRNG_RANDOM_U32;
         }
         vstr.buf[i] = rngval & 0xff;
         rngval >>= 8;
@@ -72,7 +89,6 @@ STATIC mp_obj_t mp_uos_urandom(mp_obj_t num) {
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mp_uos_urandom_obj, mp_uos_urandom);
 
 #endif // MICROPY_PY_UOS_URANDOM
-#endif // defined(MCU_SAMD51)
 
 #if MICROPY_PY_UOS_DUPTERM_BUILTIN_STREAM
 bool mp_uos_dupterm_is_builtin_stream(mp_const_obj_t stream) {
diff --git a/ports/samd/samd_isr.c b/ports/samd/samd_isr.c
index 838800bdb..a12140313 100644
--- a/ports/samd/samd_isr.c
+++ b/ports/samd/samd_isr.c
@@ -96,7 +96,7 @@ void Default_Handler(void) {
 void SysTick_Handler(void) {
     #if defined(MCU_SAMD21)
     // Use the phase jitter between the clocks to get some entropy
-    // and accumulate the random number register.
+    // and accumulate the random number register with a "spiced" LCG.
     rng_state = (rng_state * 32310901 + 1) ^ (REG_TC4_COUNT32_COUNT >> 1);
     #endif
 
@@ -108,13 +108,6 @@ void SysTick_Handler(void) {
     }
 }
 
-#if defined(MCU_SAMD21)
-uint32_t trng_random_u32(void) {
-    mp_hal_delay_ms(320); // wait for ten cycles of the rng_seed register
-    return rng_state;
-}
-#endif
-
 void us_timer_IRQ(void) {
     #if defined(MCU_SAMD21)
     if (TC4->COUNT32.INTFLAG.reg & TC_INTFLAG_OVF) {