ARM: get timer.c in, so far only with the system clock.

This test code in SysTickHanlder() should give you a rather
accurate clock with only a few seconds deviation per hour:

  #include "serial.h"
  #include "sersendf.h"
  void SysTick_Handler(void) {
    static uint32_t count = 0;
    static uint8_t minutes = 0, seconds = 0;

    count++;

    if ( ! (count % 500)) {   // A full second.
      seconds++;
      if ( ! (seconds % 60)) {
        seconds = 0;
        minutes++;
      }
      sersendf_P(PSTR("%su:"), minutes);
      if (seconds < 10)
        serial_writechar('0');
      sersendf_P(PSTR("%su\n"), seconds);
    }
  [...]

Makefile-ARM

@@ -98,7 +98,7 @@ TARGET = $(PROGRAM).hex
 # Until the generic ARM port is completed, we'd have to wrap all sources
 # in #ifdef __AVR__. To avoid this, build only a selection for now:
 SOURCES  = mendel.c cpu.c serial.c sermsg.c sersendf.c delay.c
-SOURCES += gcode_parse.c gcode_process.c pinio.c
+SOURCES += gcode_parse.c gcode_process.c pinio.c timer.c
 ifeq ($(MCU), lpc1114)
   SOURCES += cmsis-system_lpc11xx.c
 endif

mendel.c

@@ -35,8 +35,8 @@
 #include	"dda_queue.h"
 #endif /* __ARMEL_NOTYET__ */
 #include	"gcode_parse.h"
-#ifndef __ARMEL_NOTYET__
 #include	"timer.h"
+#ifndef __ARMEL_NOTYET__
 #include	"temp.h"
 #include	"watchdog.h"
 #include	"debug.h"
@@ -94,10 +94,12 @@ void init(void) {
   #if defined TEMP_MAX6675 || defined SD
     spi_init();
   #endif
+  #endif /* __ARMEL_NOTYET__ */
 
 	// set up timers
 	timer_init();
 
+  #ifndef __ARMEL_NOTYET__
 	// read PID settings from EEPROM
 	heater_init();
 

timer-arm.c

@@ -7,13 +7,56 @@
 
 #if defined TEACUP_C_INCLUDE && defined __ARMEL__
 
+#include "cmsis-core_cm0.h"
+
 
 /** Timer initialisation.
 
   Initialise timer and enable system clock interrupt. Step interrupt is
   enabled later, when we start using it.
+
+  For the system clock, we use SysTickTimer. This timer is made for exactly
+  such purposes.
 */
 void timer_init() {
+
+  /**
+    Initialise the system tick timer.
+
+    We enable the system tick timer with interrupts. A similar function is
+    SysTick_Config(uint32_t ticks) in cmsis-core_cm0.h
+
+    Register name mapping from LPC111x User Manual to CMSIS headers:
+
+      chap. 24.5     cmsis-core_cm0.h  description
+
+      SYST_CSR       SysTick->CTRL     System Timer Control and status register.
+      SYST_RVR       SysTick->LOAD     System Timer Reload value register.
+      SYST_CVR       SysTick->VAL      System Timer Current value register.
+      SYST_CALIB     SysTick->CALIB    System Timer Calibration value register.
+  */
+  NVIC_SetPriority(SysTick_IRQn, 0);              // Highest priority.
+
+  // SysTick defined in cmsis-core_cm0.h.
+  SysTick->LOAD  = TICK_TIME - 1;
+  SysTick->VAL   = 0;                             // Actually load LOAD.
+
+  SysTick->CTRL  = SysTick_CTRL_ENABLE_Msk        // Enable the ticker.
+                 | SysTick_CTRL_TICKINT_Msk       // Enable interrupt.
+                 | SysTick_CTRL_CLKSOURCE_Msk;    // Run at full CPU clock.
+}
+
+/** System clock interrupt.
+
+  Happens every TICK_TIME. Must have the same name as in
+  cmsis-startup_lpc11xx.s
+*/
+void SysTick_Handler(void) {
+
+  #ifndef __ARMEL_NOTYET__
+  clock_tick();
+  dda_clock();
+  #endif /* __ARMEL_NOTYET__ */
 }
 
 /** Specify how long until the step timer should fire.
@@ -58,6 +101,7 @@ uint8_t timer_set(int32_t delay, uint8_t check_short) {
   This means to be an emergency stop.
 */
 void timer_stop() {
+  SysTick->CTRL = 0;
 }
 
 #endif /* defined TEACUP_C_INCLUDE && defined __ARMEL__ */

timer.h

@@ -10,7 +10,7 @@
 
 /// How often we overflow and update our clock.
 /// With F_CPU = 16MHz, max is < 4.096ms (TICK_TIME = 65535).
-#define TICK_TIME 2 MS
+#define TICK_TIME (2 MS)
 
 /// Convert back to ms from cpu ticks so our system clock runs
 /// properly if you change TICK_TIME.