microbit: added accessor/mutator for systemTick

This commit adds a mutator that dynamically reconfigures the
systemTicker to call systemTick at a different period to the default.

The accessor returns the current tick speed in milliseconds.

The accessor is now also used in scheduler_tick to keep timing as
accurate as possible.

MICROBIT_DISPLAY_REFRESH_PERIOD has now been removed, and replaced with
MICROBIT_DEFAULT_TICK_PERIOD.

inc/MicroBit.h

@@ -46,12 +46,14 @@
 #define MICROBIT_NAME_CODE_LETTERS              5
 
 // Random number generator
-#define NRF51822_RNG_ADDRESS            0x4000D000
+#define NRF51822_RNG_ADDRESS                    0x4000D000
 
 
 // mbed pin assignments of core components.
-#define MICROBIT_PIN_SDA                P0_30
-#define MICROBIT_PIN_SCL                P0_0
+#define MICROBIT_PIN_SDA                        P0_30
+#define MICROBIT_PIN_SCL                        P0_0
+
+#define MICROBIT_DEFAULT_TICK_PERIOD            FIBER_TICK_PERIOD_MS
 
 /**
   * Class definition for a MicroBit device.
@@ -74,6 +76,8 @@ class MicroBit
     // Periodic callback
     Ticker                  systemTicker;
 
+    int                     tickPeriod;
+
     // I2C Interface
     MicroBitI2C             i2c;
 
@@ -259,6 +263,21 @@ class MicroBit
       */
     int removeIdleComponent(MicroBitComponent *component);
 
+
+    /*
+     * Reconfigures the ticker to the given speed in milliseconds.
+     * @param speedMs the speed in milliseconds
+     * @return MICROBIT_OK on success. MICROBIT_INVALID_PARAMETER is returned if speedUs < 1
+     *
+     * @note this will also modify the value that is added to ticks in MiroBitFiber:scheduler_tick()
+     */
+    int setTickPeriod(int speedMs);
+
+    /*
+     * Returns the currently used tick speed in milliseconds
+     */
+    int getTickPeriod();
+
     /**
       * Determine the time since this MicroBit was last reset.
       *

inc/MicroBitDisplay.h

@@ -1,12 +1,6 @@
 #ifndef MICROBIT_DISPLAY_H
 #define MICROBIT_DISPLAY_H
 
-
-/**
-  * Core Configuration settings.
-  */
-#define MICROBIT_DISPLAY_REFRESH_PERIOD     ((float)FIBER_TICK_PERIOD_MS / (float)1000)
-
 /**
   * MessageBus Event Codes
   */

source/MicroBit.cpp

@@ -1,4 +1,4 @@
-/* 
+/*
  * The underlying Nordic libraries that support BLE do not compile cleanly with the stringent GCC settings we employ
  * If we're compiling under GCC, then we suppress any warnings generated from this code (but not the rest of the DAL)
  * The ARM cc compiler is more tolerant. We don't test __GNUC__ here to detect GCC as ARMCC also typically sets this
@@ -133,8 +133,10 @@ void MicroBit::init()
     // Seed our random number generator
     seedRandom();
 
+    tickPeriod = MICROBIT_DEFAULT_TICK_PERIOD;
+
     // Start refreshing the Matrix Display
-    systemTicker.attach(this, &MicroBit::systemTick, MICROBIT_DISPLAY_REFRESH_PERIOD);
+    systemTicker.attach_us(this, &MicroBit::systemTick, tickPeriod * 1000);
 
     // Register our compass calibration algorithm.
     MessageBus.listen(MICROBIT_ID_COMPASS, MICROBIT_COMPASS_EVT_CALIBRATE, this, &MicroBit::compassCalibrator, MESSAGE_BUS_LISTENER_IMMEDIATE);
@@ -607,6 +609,35 @@ int MicroBit::removeIdleComponent(MicroBitComponent *component)
     return MICROBIT_OK;
 }
 
+/*
+ * Reconfigures the ticker to the given speed in milliseconds.
+ * @param speedMs the speed in milliseconds
+ * @return MICROBIT_OK on success. MICROBIT_INVALID_PARAMETER is returned if speedUs < 1
+ *
+ * @note this will also modify the value that is added to ticks in MiroBitFiber:scheduler_tick()
+ */
+int MicroBit::setTickPeriod(int speedMs)
+{
+    if(speedMs < 1)
+        return MICROBIT_INVALID_PARAMETER;
+
+    uBit.systemTicker.detach();
+
+    uBit.systemTicker.attach_us(this, &MicroBit::systemTick, speedMs * 1000);
+
+    tickPeriod = speedMs;
+
+    return MICROBIT_OK;
+}
+
+/*
+ * Returns the currently used tick speed in milliseconds
+ */
+int MicroBit::getTickPeriod()
+{
+    return tickPeriod;
+}
+
 /**
   * Determine the time since this MicroBit was last reset.
   *

source/MicroBitDisplay.cpp

@@ -145,7 +145,7 @@ void MicroBitDisplay::render()
 
     //timer does not have enough resolution for brightness of 1. 23.53 us
     if(brightness != MICROBIT_DISPLAY_MAXIMUM_BRIGHTNESS && brightness > MICROBIT_DISPLAY_MINIMUM_BRIGHTNESS)
-        renderTimer.attach(this, &MicroBitDisplay::renderFinish, (((float)brightness) / ((float)MICROBIT_DISPLAY_MAXIMUM_BRIGHTNESS)) * (float)MICROBIT_DISPLAY_REFRESH_PERIOD);
+        renderTimer.attach_us(this, &MicroBitDisplay::renderFinish, (((brightness * 100) / (MICROBIT_DISPLAY_MAXIMUM_BRIGHTNESS)) * uBit.getTickPeriod()));
 
     //this will take around 23us to execute
     if(brightness <= MICROBIT_DISPLAY_MINIMUM_BRIGHTNESS)

source/MicroBitFiber.cpp

@@ -181,8 +181,8 @@ void scheduler_tick()
     Fiber *t;
 
     // increment our real-time counter.
-    ticks += FIBER_TICK_PERIOD_MS;
-    
+    ticks += uBit.getTickPeriod();
+
     // Check the sleep queue, and wake up any fibers as necessary.
     while (f != NULL)
     {