purple: added PeriodicProcess library

this library provides a clean API for drivers to request periodic
timer driven calls at whatever rate they need

libraries/AP_PeriodicProcess/AP_PeriodicProcess.h

@@ -0,0 +1,9 @@
+
+#ifndef __AP_PERIODIC_PROCESS_H__
+#define __AP_PERIODIC_PROCESS_H__
+
+#include "PeriodicProcess.h"
+#include "AP_TimerProcess.h"
+#include "AP_TimerAperiodicProcess.h"
+
+#endif // __AP_PERIODIC_PROCESS_H__

libraries/AP_PeriodicProcess/AP_TimerAperiodicProcess.cpp

@@ -0,0 +1,39 @@
+
+#include "AP_TimerAperiodicProcess.h"
+
+extern "C" {
+#include <inttypes.h>
+#include <stdint.h>
+#include "WConstants.h"
+}
+// TCNT2 values for various interrupt rates,
+// assuming 256 prescaler. Note that these values
+// assume a zero-time ISR. The actual rate will be a
+// bit lower than this
+#define TCNT2_781_HZ   (256-80)
+#define TCNT2_1008_HZ  (256-62)
+#define TCNT2_1302_HZ  (256-48)
+
+uint8_t AP_TimerAperiodicProcess::_timer_offset;
+
+void AP_TimerAperiodicProcess::init( Arduino_Mega_ISR_Registry * isr_reg )
+{
+	// Enable Timer2 Overflow interrupt to trigger process.
+	TIMSK2 = 0;                 // Disable interrupts
+	TCCR2A = 0;                 // normal counting mode
+	TCCR2B = _BV(CS21) | _BV(CS22);	// Set prescaler of clk/256
+	TCNT2  = 0;                 // Set count to zero, so it goes off right away.
+	TIFR2  = _BV(TOV2);	        // clear pending interrupts;
+	TIMSK2 = _BV(TOIE2);        // enable the overflow interrupt
+
+    isr_reg->register_signal(ISR_REGISTRY_TIMER2_OVF, AP_TimerAperiodicProcess::run);
+}
+
+void AP_TimerAperiodicProcess::run(void)
+{
+    _timer_offset = (_timer_offset + 49) % 32;
+    _period = TCNT2_781_HZ + _timer_offset;
+    TCNT2 = _period;
+    if (_proc != NULL)
+        _proc();
+}

libraries/AP_PeriodicProcess/AP_TimerAperiodicProcess.h

@@ -0,0 +1,19 @@
+
+#ifndef __AP_TIMER_APERIODIC_PROCESS_H__
+#define __AP_TIMER_APERIODIC_PROCESS_H__
+
+#include <stdint.h>
+
+#include "../Arduino_Mega_ISR_Registry/Arduino_Mega_ISR_Registry.h"
+#include "AP_TimerProcess.h"
+
+class AP_TimerAperiodicProcess : public AP_TimerProcess
+{
+    public:
+        void init( Arduino_Mega_ISR_Registry * isr_reg );
+        static void run(void);
+    private:
+        static uint8_t _timer_offset;
+};
+
+#endif // __AP_TIMER_APERIODIC_PROCESS_H__

libraries/AP_PeriodicProcess/AP_TimerProcess.cpp

@@ -0,0 +1,43 @@
+
+#include "AP_TimerProcess.h"
+
+extern "C" {
+#include <inttypes.h>
+#include <stdint.h>
+#include "WConstants.h"
+}
+
+int AP_TimerProcess::_period;
+void (*AP_TimerProcess::_proc)(void);
+
+AP_TimerProcess::AP_TimerProcess(int period)
+{
+    _period = period;
+    _proc = NULL;
+}
+
+void AP_TimerProcess::init( Arduino_Mega_ISR_Registry * isr_reg )
+{
+	// Enable Timer2 Overflow interrupt to trigger process.
+	TIMSK2 = 0;                 // Disable interrupts
+	TCCR2A = 0;                 // normal counting mode
+	TCCR2B = _BV(CS21) | _BV(CS22);	// Set prescaler of clk/256
+	TCNT2  = 0;                 // Set count to zero, so it goes off right away.
+	TIFR2  = _BV(TOV2);	        // clear pending interrupts;
+	TIMSK2 = _BV(TOIE2);        // enable the overflow interrupt
+
+    isr_reg->register_signal( ISR_REGISTRY_TIMER2_OVF, AP_TimerProcess::run);
+}
+
+void AP_TimerProcess::register_process(void (*proc)(void) )
+{
+    _proc = proc;
+    TCNT2 = 1;                 // Should go off almost immediately.
+}
+
+void AP_TimerProcess::run(void)
+{
+    TCNT2 = _period;
+    if (_proc != NULL)
+        _proc();
+}

libraries/AP_PeriodicProcess/AP_TimerProcess.h

@@ -0,0 +1,23 @@
+
+#ifndef __AP_TIMERPROCESS_H__
+#define __AP_TIMERPROCESS_H__
+
+#include "PeriodicProcess.h"
+#include "../Arduino_Mega_ISR_Registry/Arduino_Mega_ISR_Registry.h"
+
+/* XXX this value is a total guess, will look up. */
+#define TIMERPROCESS_PER_DEFAULT (256)
+
+class AP_TimerProcess : public AP_PeriodicProcess
+{
+    public:
+        AP_TimerProcess(int period = TIMERPROCESS_PER_DEFAULT);
+        void init( Arduino_Mega_ISR_Registry * isr_reg );
+        void register_process(void (* proc)(void));
+        static void run(void);
+    protected:
+        static int  _period;
+        static void (*_proc)(void);
+};
+
+#endif // __AP_TIMERPROCESS_H__

libraries/AP_PeriodicProcess/PeriodicProcess.h

@@ -0,0 +1,11 @@
+
+#ifndef __PERIODICPROCESS_H__
+#define __PERIODICPROCESS_H__
+
+class AP_PeriodicProcess
+{
+    public:
+        virtual void register_process(void (* proc)(void)) = 0;
+};
+
+#endif // __PERIODICPROCESS_H__