LoRa/SX1276: adjust Rx window offset and duration

After a transmission, two receive windows Rx1 and Rx2 are opened
for one second each, one right after the other, after a fixed delay
(for example 5s). The Rx window offset is added to the starting date
of each window and the Rx window duration represents the maximum
delay we will wait for an incoming message before declaring that
a timeout occurred.

A value of -500ms for the offset and 800ms for the duration means
that instead of having Rx1 = [5000, 6000[ and Rx2 = [6000, 7000[
we get Rx1 = [4500, 5300[ and Rx2 = [5500, 6300[. We only cover
30% of the expected windows.

The maximum time a SX127x can take before the Rx side is ready is
TS_HOP + TS_RE = 50µs + 2.33ms. Using 3ms for the offset and
1003ms for the duration will give much better time windows:
Rx1 = [4997, 5997[ and Rx2 = [5997, 7000]. Note that the
lorawan-device crate caps Rx1 end date to Rx2 start date.

This change allows a previously failing Murata CMWX1ZZABZ-091
module (STM32L + SX1276) to connect to the TTN LoRa network.

1 files changed, 2 insertions, 2 deletions

diff --git a/embassy-lora/src/sx127x/mod.rs b/embassy-lora/src/sx127x/mod.rs
index f47a9eb55..8904c9a13 100644
--- a/embassy-lora/src/sx127x/mod.rs
+++ b/embassy-lora/src/sx127x/mod.rs
@@ -70,10 +70,10 @@ where
     RFS: RadioSwitch + 'static,
 {
     fn get_rx_window_offset_ms(&self) -> i32 {
-        -500
+        -3
     }
     fn get_rx_window_duration_ms(&self) -> u32 {
-        800
+        1003
     }
 }