samd/machine_bitstream: Tune ticks for SAMD51 and set pin to output.

Changes in this commit:
- Change ticks overhead value for SAMD51.  The value was too large and thus
  timing was quite bad.  At 120Mhz it's now within +/- 30ns.
- Set the pin to output mode.  That way, all Pin identifiers are accepted
  as argument, not only Pin objects.

Tested with ItsyBitsy M4 and M0 boards.

Signed-off-by: robert-hh <robert@hammelrath.com>

Author: robert-hh

ports/samd/machine_bitstream.c

@@ -36,7 +36,7 @@
 
 // No cycle counter on M0, do manual cycle counting instead.
 
-// STM32F091 @ 48MHz
+// SAMD21 @ 48MHz
 #define NS_CYCLES_PER_ITER_HIGH (3)
 #define NS_CYCLES_PER_ITER_LOW (3)
 #define NS_OVERHEAD_CYCLES_HIGH (12)
@@ -52,6 +52,7 @@ void machine_bitstream_high_low(mp_hal_pin_obj_t pin, uint32_t *timing_ns, const
     volatile const uint32_t mask = 1 << (pin % 32);
     volatile uint32_t *outclr = &PORT->Group[pin / 32].OUTCLR.reg;
     volatile uint32_t *outset = &PORT->Group[pin / 32].OUTSET.reg;
+    mp_hal_pin_output(pin);
 
     // Convert ns to loop iterations [high_time_0, low_time_0, high_time_1, low_time_1].
     for (size_t i = 0; i < 4; ++i) {
@@ -163,11 +164,12 @@ void machine_bitstream_high_low(mp_hal_pin_obj_t pin, uint32_t *timing_ns, const
 
 #else // > CORTEX_M0
 
-#define NS_TICKS_OVERHEAD (70)
+#define NS_TICKS_OVERHEAD (40)
 
 void machine_bitstream_high_low(mp_hal_pin_obj_t pin, uint32_t *timing_ns, const uint8_t *buf, size_t len) {
     uint32_t fcpu_mhz = get_cpu_freq() / 1000000;
     uint32_t ticks_overhead = fcpu_mhz * NS_TICKS_OVERHEAD / 1000;
+    mp_hal_pin_output(pin);
     // Convert ns to us ticks [high_time_0, period_0, high_time_1, period_1].
     for (size_t i = 0; i < 4; ++i) {
         timing_ns[i] = fcpu_mhz * timing_ns[i] / 1000;
@@ -191,8 +193,8 @@ void machine_bitstream_high_low(mp_hal_pin_obj_t pin, uint32_t *timing_ns, const
             mp_hal_pin_high(pin);
             while ((mp_hal_ticks_cpu() - start_ticks) < t[0]) {
             }
-            b <<= 1;
             mp_hal_pin_low(pin);
+            b <<= 1;
             while ((mp_hal_ticks_cpu() - start_ticks) < t[1]) {
             }
         }