Fix #442: re-adjust window calcuations, expand jitter, fix clock erro…

…r, so we arrive on-time

src/lmic/lmic.c

@@ -1476,16 +1476,22 @@ ostime_t LMICcore_adjustForDrift (ostime_t delay, ostime_t hsym, rxsyms_t rxsyms
     // If the clock is slow, the window needs to open earlier in our time
     // in order to open at or before the specified time (in real world),.
     // Don't bother to round, as this is very fine-grained.
-    // and bear in mind that the delay is always
     ostime_t drift = (ostime_t)(((int64_t)delay * clockerr) / MAX_CLOCK_ERROR);
 
-    // we add symbols if we're starting tx before the window.
-    rxoffset -= drift;
-
-    // if we're starting before the window, extend the number of symbols
-    // for timeout appropriately.
+    // calculate the additional rxsyms needed to hit the window nominally.
+    ostime_t const tsym = 2 * hsym;
+    ostime_t driftwin;
+    driftwin = 2 * drift;
     if (rxoffset < 0)
-        rxsyms_in += (-rxoffset + 2 * hsym - 1) / (2 * hsym);
+        driftwin += -rxoffset;
+    // else we'll hit the window nominally.
+
+    rxsyms_in += (driftwin + tsym - 1) / tsym;
+
+    // reduce the rxoffset by the drift; this compensates for a slow clock;
+    // but it makes the rxtime too early by approximately `drift` if clock
+    // is fast.
+    rxoffset -= drift;
 
     setRxsyms(rxsyms_in);
 

src/lmic/lmic.h

@@ -331,13 +331,14 @@ enum {
         MAX_CLOCK_ERROR = 65536,
         //! \brief maximum clock error that users can specify: 2000 ppm (0.2%).
         //! \details This is the limit for clock error, unless LMIC_ENABLE_arbitrary_clock_error is set.
-        //! The default is 2,000 ppm, which is .002, or 0.2%. If your clock error is bigger,
+        //! The default is 4,000 ppm, which is .004, or 0.4%; this is what you get on an
+        //! STM32L0 running with the HSI oscillator after cal. If your clock error is bigger,
         //! usually you want to calibrate it so that millis() and micros() are reasonably
         //! accurate. Important: do not use clock error to compensate for late serving
         //! of the LMIC. If you see that LMIC.radio.rxlate_count is increasing, you need
         //! to adjust your application logic so the LMIC gets serviced promptly when a
         //! Class A downlink (or beacon) is pending.
-        LMIC_kMaxClockError_ppm = 2000,
+        LMIC_kMaxClockError_ppm = 4000,
 };
 
 // callbacks for client alerts.

src/lmic/oslmic.h

@@ -124,15 +124,15 @@ void radio_monitor_rssi(ostime_t n, oslmic_radio_rssi_t *pRssi);
 // to platform issues. It's specified in units of ostime_t. It must reflect
 // platform jitter and latency, as well as the speed of the LMIC when running
 // on this plaform.
-#define RX_RAMPUP  (us2osticks(2000))
+#define RX_RAMPUP  (us2osticks(10000))
 #endif
 
 #ifndef TX_RAMPUP
 // TX_RAMPUP specifies the extra time we must allow to set up a TX event) due
 // to platform issues. It's specified in units of ostime_t. It must reflect
 // platform jitter and latency, as well as the speed of the LMIC when running
 // on this plaform.
-#define TX_RAMPUP  (us2osticks(2000))
+#define TX_RAMPUP  (us2osticks(10000))
 #endif
 
 #ifndef OSTICKS_PER_SEC

src/lmic/radio.c

@@ -902,6 +902,18 @@ static CONST_TABLE(u1_t, rxlorairqmask)[] = {
     [RXMODE_RSSI]   = 0x00,
 };
 
+//! \brief handle late RX events.
+//! \param nLate is the number of `ostime_t` ticks that the event was late.
+//! \details If nLate is non-zero, increment the count of events, totalize
+//! the number of ticks late, and (if implemented) adjust the estimate of
+//! what would be best to return from `os_getRadioRxRampup()`.
+static void rxlate (u4_t nLate) {
+    if (nLate) {
+            LMIC.radio.rxlate_ticks += nLate;
+            ++LMIC.radio.rxlate_count;
+    }
+}
+
 // start LoRa receiver (time=LMIC.rxtime, timeout=LMIC.rxsyms, result=LMIC.frame[LMIC.dataLen])
 static void rxlora (u1_t rxmode) {
     // select LoRa modem (from sleep mode)
@@ -966,13 +978,9 @@ static void rxlora (u1_t rxmode) {
     // now instruct the radio to receive
     if (rxmode == RXMODE_SINGLE) { // single rx
         u4_t nLate = hal_waitUntil(LMIC.rxtime); // busy wait until exact rx time
-        LMICOS_logEventUint32("+Rx LoRa Single", nLate);
         opmode(OPMODE_RX_SINGLE);
-        if (nLate)
-            {
-            ++LMIC.radio.rxlate_count;
-            LMIC.radio.rxlate_ticks += nLate;
-            }
+        LMICOS_logEventUint32("+Rx LoRa Single", nLate);
+        rxlate(nLate);
 #if LMIC_DEBUG_LEVEL > 0
         ostime_t now = os_getTime();
         LMIC_DEBUG_PRINTF("start single rx: now-rxtime: %"LMIC_PRId_ostime_t"\n", now - LMIC.rxtime);
@@ -1042,12 +1050,9 @@ static void rxfsk (u1_t rxmode) {
     // now instruct the radio to receive
     if (rxmode == RXMODE_SINGLE) {
         u4_t nLate = hal_waitUntil(LMIC.rxtime); // busy wait until exact rx time
-        LMICOS_logEventUint32("+Rx FSK", nLate);
         opmode(OPMODE_RX); // no single rx mode available in FSK
-        if (nLate) {
-            LMIC.radio.rxlate_ticks += nLate;
-            ++LMIC.radio.rxlate_count;
-        }
+        LMICOS_logEventUint32("+Rx FSK", nLate);
+        rxlate(nLate);
     } else {
         LMICOS_logEvent("+Rx FSK Continuous");
         opmode(OPMODE_RX);