X-Git-Url: https://oss.titaniummirror.com/gitweb/?a=blobdiff_plain;f=tos%2Fplatforms%2Ftelosb%2Fmac%2Ftkn154%2Ftimer%2FAlarm32khzTo62500hzTransformC.nc;h=2af284dd9d1e3375c11b87da5735fdad8444a625;hb=82c215ff867ea7ef4fa748fb72b686c32ffb25e1;hp=20f0822d49e1d7863038d7f834e28b9ad3770734;hpb=afb6da3df34fdebb6843598761ef4de5eb0186f6;p=tinyos-2.x.git

diff --git a/tos/platforms/telosb/mac/tkn154/timer/Alarm32khzTo62500hzTransformC.nc b/tos/platforms/telosb/mac/tkn154/timer/Alarm32khzTo62500hzTransformC.nc
index 20f0822d..2af284dd 100644
--- a/tos/platforms/telosb/mac/tkn154/timer/Alarm32khzTo62500hzTransformC.nc
+++ b/tos/platforms/telosb/mac/tkn154/timer/Alarm32khzTo62500hzTransformC.nc
@@ -41,10 +41,13 @@ module Alarm32khzTo62500hzTransformC
 implementation
 {
 /**
- * This is the place where we cheat: since we don't have a clock source
- * running at 62500 Hz, we cast 2 symbols to 1 tick of the 32768
- * clock, which introduces a small (5%) error.
-*/
+ * TelosB lacks a clock with the precision and accuracy 
+ * required by the 802.15.4 standard (62500 Hz, 40 ppm).
+ * As a workaround, we cast one tick of the 32768 Hz clock to 
+ * two 802.15.4 symbols, which introduces a small (5%) error.
+ * Thus the channel access in particular in beacon-enabled PANs 
+ * (slotted CSMA-CA) is not be standard-compliant!
+ */
 #warning "Warning: MAC timing is not standard compliant (the symbol clock is based on the 32768 Hz oscillator)!"
 
   async command void Alarm.start[ uint8_t num ](uint32_t dt){ call AlarmFrom.start[num](dt >> 1);}
@@ -61,15 +64,17 @@ implementation
 
   async command void Alarm.startAt[ uint8_t num ](uint32_t t0, uint32_t dt){
     // t0 occured before "now"
-    uint32_t now = call Alarm.getNow[num](), elapsed;
-    if (t0 <= now)
-      elapsed = now - t0;
-    else
-      elapsed = ~(t0 - now) + 1;
-    if (elapsed > dt)
-      elapsed = dt;
-    dt -= elapsed;
-    call AlarmFrom.start[num](dt >> 1);
+    atomic {
+      uint32_t now = call Alarm.getNow[num](), elapsed;
+      if (t0 < now)
+        elapsed = now - t0;
+      else
+        elapsed = ~(t0 - now) + 1;
+      if (elapsed > dt)
+        dt = elapsed;
+      dt -= elapsed;
+      call Alarm.start[num](dt);
+    }
   }
 
   /******************** Defaults ****************************/