samd/moduos: Add uos.urandom() for SAMD51.

Based on the hardware RNG.

ports/samd/boards/mpconfig_samd51.h

@@ -5,6 +5,9 @@
 #define MICROPY_PY_BUILTINS_COMPLEX     (0)
 #define MICROPY_PY_MATH                 (0)
 #define MICROPY_PY_CMATH                (0)
+#define MICROPY_PY_UOS_URANDOM          (1)
+#define MICROPY_PY_URANDOM_SEED_INIT_FUNC (trng_random_u32())
+unsigned long trng_random_u32(void);
 
 // Due to a limitation in the TC counter for us, the ticks period is 2**29
 #define MICROPY_PY_UTIME_TICKS_PERIOD   (0x20000000)

ports/samd/moduos.c

@@ -34,6 +34,46 @@
 #include "modmachine.h"
 #include "sam.h"
 
+#if defined(MCU_SAMD51)
+static bool initialized = false;
+
+STATIC void trng_start(void) {
+    if (!initialized) {
+        MCLK->APBCMASK.bit.TRNG_ = 1;
+        REG_TRNG_CTRLA = TRNG_CTRLA_ENABLE;
+        initialized = true;
+    }
+}
+
+uint32_t trng_random_u32(void) {
+    trng_start();
+    while ((REG_TRNG_INTFLAG & TRNG_INTFLAG_DATARDY) == 0) {
+    }
+    return REG_TRNG_DATA;
+}
+
+#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);
+    vstr_t vstr;
+    vstr_init_len(&vstr, n);
+    uint32_t rngval = 0;
+
+    trng_start();
+    for (int i = 0; i < n; i++) {
+        if ((i % 4) == 0) {
+            rngval = trng_random_u32();
+        }
+        vstr.buf[i] = rngval & 0xff;
+        rngval >>= 8;
+    }
+    return mp_obj_new_bytes_from_vstr(&vstr);
+}
+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) {
     const mp_obj_type_t *type = mp_obj_get_type(stream);