STM32F411: get delay-stm32.c in

delay-lpc.c

@@ -3,7 +3,7 @@
   \brief Delay routines, ARM specific part.
 */
 
-#if defined TEACUP_C_INCLUDE && defined __ARMEL__
+#if defined TEACUP_C_INCLUDE && defined __ARM_LPC1114__
 
 #include "cmsis-lpc11xx.h"  // For __ASM() and __SYSTEM_CLOCK.
 
@@ -52,4 +52,4 @@ void delay_us(uint16_t delay) {
   #endif
 }
 
-#endif /* defined TEACUP_C_INCLUDE && defined __ARMEL__ */
+#endif /* defined TEACUP_C_INCLUDE && defined __ARM_LPC1114__ */

delay-stm32.c

@@ -0,0 +1,50 @@
+
+/** \file
+  \brief Delay routines, ARM specific part.
+*/
+
+#if defined TEACUP_C_INCLUDE && defined __ARM_STM32F411__
+
+#include "mbed-stm32f4xx.h"  // For __ASM() and ...
+
+
+/** Delay in microseconds.
+
+  \param delay Time to wait in microseconds.
+
+  Execution times on ARM aren't as predictable as they could be, because
+  there's a code prefetch engine which can change timings depending on the
+  position of the code in Flash. We could use the System Tick Timer for this
+  task, but this timer is probably better used for more important tasks.
+  delay_us() and delay_ms() are used only rarely and not in a way which would
+  require high precision.
+
+  Nevertheless, calibrated on the oscilloscope. Measured accuracy:
+
+             delay_us(10)    ...(100)   ...(1000)  ...(10000)  ...(65000)
+    96 MHz       10.23 us    99.3 us    0.994 ms    9.93 ms    64.5 ms
+
+  CAUTION: this currently works for a 96 MHz clock, only! As other clock rates
+           appear, there's more math neccessary, see the AVR version. Or simply
+           a second implementation.
+*/
+void delay_us(uint16_t delay) {
+
+    __ASM (".balign 16");  // No gambling with the prefetch engine.
+    while (delay) {
+      __ASM volatile (
+        "   nop               \n\t"  // One nop before the loop slows about 2%.
+        "   movs  r7, #9      \n\t"  // One more loop round slows about 20%
+        "1: nop               \n\t"
+        "   sub   r7, #1      \n\t"
+        "   cmp   r7, #0      \n\t"
+        "   bne   1b          \n\t"
+        :
+        :
+        : "r7", "cc"
+      );
+      delay--;
+    }
+}
+
+#endif /* defined TEACUP_C_INCLUDE && defined __ARM_STM32F411__ */

delay.c

@@ -8,6 +8,7 @@
 #include "delay-avr.c"
 // Each ARM needs it's own file
 #include "delay-lpc.c"
+#include "delay-stm32.c"
 #undef TEACUP_C_INCLUDE
 
 #include "watchdog.h"
@@ -19,7 +20,8 @@
 
   Accuracy on AVR, 20 MHz: delay < 0.04% too long over the whole range.
   Accuracy on AVR, 16 MHz: delay < 0.8% too short over the whole range.
-  Accuracy on ARM, 48 MHz: delay < 0.1% too long over the whole range.
+  Accuracy on LPC1114, 48 MHz: delay < 0.1% too long over the whole range.
+  Accuracy on STM32F411, 96 MHz: delay < 0.7% too short over the whole range.
 */
 void delay_ms(uint32_t delay) {
 	wd_reset();