2009-03-02 Till Straumann <strauman@slac.stanford.edu>

	PR 1375/bsps
	* clock/clock.c: Correct implementation of nanoseconds since last tick
	handler.

2 files changed, 64 insertions, 5 deletions

diff --git a/c/src/lib/libbsp/m68k/uC5282/ChangeLog b/c/src/lib/libbsp/m68k/uC5282/ChangeLog
index 24ec642801..579552a7c9 100644
--- a/c/src/lib/libbsp/m68k/uC5282/ChangeLog
+++ b/c/src/lib/libbsp/m68k/uC5282/ChangeLog
@@ -1,3 +1,9 @@
+2009-03-02	Till Straumann <strauman@slac.stanford.edu>
+
+	PR 1375/bsps
+	* clock/clock.c: Correct implementation of nanoseconds since last tick
+	handler.
+
 2009-02-25	Joel Sherrill <joel.sherrill@OARcorp.com>
 
 	* clock/clock.c, include/bsp.h: Make bsp idle thread names match.
diff --git a/c/src/lib/libbsp/m68k/uC5282/clock/clock.c b/c/src/lib/libbsp/m68k/uC5282/clock/clock.c
index 6ea98c875f..a15d1536f6 100644
--- a/c/src/lib/libbsp/m68k/uC5282/clock/clock.c
+++ b/c/src/lib/libbsp/m68k/uC5282/clock/clock.c
@@ -35,13 +35,66 @@ extern int __SRAMBASE[];
 
 uint32_t bsp_clock_nanoseconds_since_last_tick(void)
 {
-    int i = MCF5282_PIT3_PCNTR;
-    if (MCF5282_PIT3_PCSR & MCF5282_PIT_PCSR_PIF)
-        i = MCF5282_PIT3_PCNTR - USEC_PER_TICK;
-    return (USEC_PER_TICK - i) * 1000;
+  uint32_t    tdiff;
+
+  /* Details to consider here:
+   *
+   *  - PIT is 16-bit. To properly handle differences of two
+   *    16-bit numbers (which requires 17-bits) we want to
+   *    do the arithmetic in a wider data type. PIT reading
+   *    is a unsigned 16-bit count.
+   *
+   *  => must make sure PIT is a *unsigned* 16-bit type. Otherwise
+   *     values get sign-extended when converted to wider type
+   *     (regardless of the signedness of wider type):
+   *
+   *       (unsigned)(signed short)0xffff -> 0xffffffff.
+   *
+   *     and thus 
+   *
+   *       (uint32_t)(int16_t)65535 - (uin32_t)(int16_t)1 
+   *
+   *     would yield 0xfffffffe, not 65534!
+   *   
+   *
+   *  - PIT counts backwards from PMR -> zero, hence
+   *
+   *      now - tick_base = (PMR - now) - (PMR - tick_base) = tick_base - now;
+   * 
+   *    result may be negative (if rolled-over).
+   *
+   *  - PIF flag, counter and PCNTR_AT_TICK must all
+   *    be read atomically - otherwise an interrupt may
+   *    have altered their values while we're looking.
+   *
+   *    NOTE: score framework calling this routine disables
+   *          interrupts during execution of this callout.
+   *
+   *  - Last but not least, rollover might have happened
+   *    just between reading counter and PIF flag; hence
+   *    we have to re-read the counter if PIF is set.
+   *
+   */
+
+  /* obtain current value */
+  tdiff     = (uint16_t)MCF5282_PIT3_PCNTR;
+
+  if (MCF5282_PIT3_PCSR & MCF5282_PIT_PCSR_PIF) {
+    /* rollover may just have happened;
+     * must reload PCNTR.
+     */
+    tdiff = + (uint32_t)(uint16_t)MCF5282_PIT3_PMR
+            + (uint32_t)(uint16_t)PCNTR_AT_TICK
+            - (uint32_t)(uint16_t)MCF5282_PIT3_PCNTR;
+  } else {
+    tdiff = + (uint32_t)(uint16_t)PCNTR_AT_TICK - tdiff;
+  }
+
+  return tdiff * 1000;
 }
 
-#define Clock_driver_nanoseconds_since_last_tick bsp_clock_nanoseconds_since_last_tick
+#define Clock_driver_nanoseconds_since_last_tick \
+    bsp_clock_nanoseconds_since_last_tick
 
 /*
  * Periodic interval timer interrupt handler