extmod/machine_pulse: Make time_pulse_us() not throw exceptions.

machine.time_pulse_us() is intended to provide very fine timing, including
while working with signal bursts, where each transition is tracked in row.
Throwing and handling an exception may take too much time and "signal loss".
So instead, in case of a timeout, just return negative value. Cases of
timeout while waiting for initial signal stabilization, and during actual
timing, are recognized.

The documentation is updated accordingly, and rewritten somewhat to clarify
the function behavior.

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/dht/dht.c b/drivers/dht/dht.c
index 1f0cffc6f..6bdda44b4 100644
--- a/drivers/dht/dht.c
+++ b/drivers/dht/dht.c
@@ -65,7 +65,7 @@ STATIC mp_obj_t dht_readinto(mp_obj_t pin_in, mp_obj_t buf_in) {
 
     // time pulse, should be 80us
     ticks = machine_time_pulse_us(pin, 1, 150);
-    if (ticks == (mp_uint_t)-1) {
+    if ((mp_int_t)ticks < 0) {
         goto timeout;
     }
 
@@ -73,7 +73,7 @@ STATIC mp_obj_t dht_readinto(mp_obj_t pin_in, mp_obj_t buf_in) {
     uint8_t *buf = bufinfo.buf;
     for (int i = 0; i < 40; ++i) {
         ticks = machine_time_pulse_us(pin, 1, 100);
-        if (ticks == (mp_uint_t)-1) {
+        if ((mp_int_t)ticks < 0) {
             goto timeout;
         }
         buf[i / 8] = (buf[i / 8] << 1) | (ticks > 48);