Port logic to enable PCK6 in tickless mode from the samv71-xult to the same70-xplained

configs/same70-xplained/README.txt

@@ -921,8 +921,8 @@ Tickless OS
   will have an error of 0.6%  and will have inaccuracies that will
   effect the time due to long term error build-up.
 
-  UPDATE: As of this writing (2015-12-03), the Tickless support is
-  functional.  However, there are inaccuracies  in delays.  For example,
+  Using the slow clock clock input, the Tickless support is functional,
+  however, there are inaccuracies  in delays.  For example,
 
     nsh> sleep 10
 
@@ -936,10 +936,18 @@ Tickless OS
   piece has a large error in the calculation.  The cumulative error is the
   cause of the problem.
 
-  Solution:  30.518 microseconds is not representable and the value of
-  CONFIG_USEC_PER_SEC is too inaccurate;  error build-up is throwing off
-  long delays (short delays are probably still okay).  We should driver
-  the interval timer with PCK6 with a period that is exactly representable.
+  Solution:  The same70-xplained/src/sam_boot.c file has additional logic
+  to enable the programmable clock PCK6 as a clock source for the
+  timer/counters if the Tickless mode is selected.  The ideal frequency
+  would be:
+
+    frequency = 1,000,000 / CONFIG_USEC_PER_TICK
+
+  The main crystal is selected as the frequency source.  The maximum
+  prescaler value is 256 so the minimum frequency is 46,875 Hz which
+  corresponds to a period of 21.3 microseconds.  A value of
+  CONFIG_USEC_PER_TICK=20, or 50KHz, would give an exact solution with
+  a divider of 240.
 
   SAME70 Timer Usage
   ------------------

configs/same70-xplained/src/sam_boot.c

@@ -39,23 +39,20 @@
 
 #include <nuttx/config.h>
 
+#include <stdint.h>
+#include <stdbool.h>
+#include <assert.h>
 #include <debug.h>
 
 #include <nuttx/board.h>
+#include <nuttx/clock.h>
 #include <arch/board/board.h>
 
 #include "up_arch.h"
 #include "sam_start.h"
+#include "sam_pck.h"
 #include "same70-xplained.h"
 
-/****************************************************************************
- * Pre-processor Definitions
- ****************************************************************************/
-
-/****************************************************************************
- * Private Functions
- ****************************************************************************/
-
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
@@ -73,6 +70,37 @@
 
 void sam_boardinitialize(void)
 {
+#ifdef CONFIG_SCHED_TICKLESS
+  uint32_t frequency;
+  uint32_t actual;
+
+  /* If Tickless mode is selected then enabled PCK6 as a possible clock
+   * source for the timer/counters.  The ideal frequency could be:
+   *
+   *  frequency = 1,000,000 / CONFIG_USEC_PER_TICK
+   *
+   * The main crystal is selected as the frequency source.  The maximum
+   * prescaler value is 256 so the minimum frequency is 46,875 Hz which
+   * corresponds to a period of 21.3 microseconds.  A value of
+   * CONFIG_USEC_PER_TICK=20, or 50KHz, would give an exact solution with
+   * a divider of 240.
+   */
+
+  frequency = USEC_PER_SEC / CONFIG_USEC_PER_TICK;
+  DEBUGASSERT(frequency >= (BOARD_MAINOSC_FREQUENCY / 256));
+
+  actual = sam_pck_configure(PCK6, PCKSRC_MAINCK, frequency);
+
+  /* We expect to achieve this frequency exactly */
+
+  DEBUGASSERT(actual == frequency);
+  UNUSED(actual);
+
+  /* Enable PCK6 */
+
+  (void)sam_pck_enable(PCK6, true);
+#endif
+
 #ifdef CONFIG_SAMV7_SDRAMC
   /* Configure SDRAM if it has been enabled in the NuttX configuration.
    * Here we assume, of course, that we are not running out SDRAM.

configs/samv71-xult/README.txt

@@ -1506,8 +1506,8 @@ Tickless OS
   will have an error of 0.6%  and will have inaccuracies that will
   effect the time due to long term error build-up.
 
-  UPDATE: As of this writing (2015-12-03), the Tickless support is
-  functional.  However, there are inaccuracies  in delays.  For example,
+  Using the slow clock clock input, the Tickless support is functional,
+  however, there are inaccuracies  in delays.  For example,
 
     nsh> sleep 10
 
@@ -1521,10 +1521,18 @@ Tickless OS
   piece has a large error in the calculation.  The cumulative error is the
   cause of the problem.
 
-  Solution:  30.518 microseconds is not representable and the value of
-  CONFIG_USEC_PER_SEC is too inaccurate;  error build-up is throwing off
-  long delays (short delays are probably still okay).  We should driver
-  the interval timer with PCK6 with a period that is exactly representable.
+  Solution:  The samv71-xult/src/sam_boot.c file has additional logic
+  to enable the programmable clock PCK6 as a clock source for the
+  timer/counters if the Tickless mode is selected.  The ideal frequency
+  would be:
+
+    frequency = 1,000,000 / CONFIG_USEC_PER_TICK
+
+  The main crystal is selected as the frequency source.  The maximum
+  prescaler value is 256 so the minimum frequency is 46,875 Hz which
+  corresponds to a period of 21.3 microseconds.  A value of
+  CONFIG_USEC_PER_TICK=20, or 50KHz, would give an exact solution with
+  a divider of 240.
 
   SAMV7 Timer Usage
   -----------------

configs/samv71-xult/src/sam_boot.c

@@ -53,14 +53,6 @@
 #include "sam_pck.h"
 #include "samv71-xult.h"
 
-/****************************************************************************
- * Pre-processor Definitions
- ****************************************************************************/
-
-/****************************************************************************
- * Private Functions
- ****************************************************************************/
-
 /****************************************************************************
  * Public Functions
  ****************************************************************************/