updating blinker02

ATSAMD21/blinker02/README

@@ -34,7 +34,21 @@ allow us to modify the OSC8M register.
 By simply changing the prescaler we can adjust the speed of the clock.
 
 This example will do three on/off cycles at 16 seconds per cycle, then
-then from there the leds will change state every 1 second.
+then 7 on/off cycles at 2 seconds per cycle.  Then I have some
+code that shows another way to use a timer that wraps around.  By
+changing the load value to all ones (this is a 24 bit timer) it counts
+from 0xFFFFFF to 0x000000 and then "wraps around" to 0xFFFFFF which
+means it counts forever.  I know it is a down counter because the ARM
+docs said that, you have to check some timers are up some are down
+some you can configure to be up or down.  So long as you dont wait
+longer than a single wrap around (with some margin) you can simply
+compare the difference between reading the timer one time and then
+reading it later to see how much time has elapsed.  The last loop
+demonstrates this.  And is the most generic as you often find a timer
+can be setup to wrap around and you dont have to get into the details
+of that specific timer to figure out how to check that it has finished
+counting, how to re-arm it, etc.  Any wraparound timer you can just
+read it once then read it again and subtract (and mask).
 
 Remember this is not a crystal oscillator, it is not that accurate.
 The docs say that there is calibration data (meaning each individual

ATSAMD21/blinker02/blinker02.c

@@ -44,6 +44,7 @@ int delay ( unsigned int n )
 int notmain ( void )
 {
     unsigned int ra;
+    unsigned int rb;
 
 if(1)
 {
@@ -81,7 +82,9 @@ if(1)
     ra&=~(3<<8);
     PUT32(OSC8M,ra);
 
-    while(1)
+if(1)
+{
+    for(ra=0;ra<7;ra++)
     {
         PUT32(PORTAOUTSET,1<<17);
         PUT32(PORTAOUTSET,1<<27);
@@ -92,6 +95,35 @@ if(1)
         PUT32(PORTBOUTCLR,1<<3);
         delay(8);
     }
+}
+
+    PUT32(STK_CSR,4);
+    PUT32(STK_RVR,0x00FFFFFF);
+    PUT32(STK_CVR,0x00000000);
+    PUT32(STK_CSR,5);
+
+    rb=GET32(STK_CVR);
+    while(1)
+    {
+        PUT32(PORTAOUTSET,1<<17);
+        PUT32(PORTAOUTSET,1<<27);
+        PUT32(PORTBOUTSET,1<<3);
+        while(1)
+        {
+            ra=GET32(STK_CVR);
+            if(((rb-ra)&STK_MASK)>=1234567) break;
+        }
+        rb=ra;
+        PUT32(PORTAOUTCLR,1<<17);
+        PUT32(PORTAOUTCLR,1<<27);
+        PUT32(PORTBOUTCLR,1<<3);
+        while(1)
+        {
+            ra=GET32(STK_CVR);
+            if(((rb-ra)&STK_MASK)>=1234567) break;
+        }
+        rb=ra;
+    }
 
     return(0);
 }