From 7722eb9cc94d8538758ced9153f2fac5b75bdce1 Mon Sep 17 00:00:00 2001
From: Thibaut VIARD <thibaut.viard@atmel.com>
Date: Wed, 17 Sep 2014 17:17:22 +0200
Subject: [PATCH] Fixing delayMicroseconds()
---
cores/arduino/delay.h | 59 +++++++++++++++++++++++--------------------
1 file changed, 31 insertions(+), 28 deletions(-)
diff --git a/cores/arduino/delay.h b/cores/arduino/delay.h
index 9809fc37..527bf464 100644
--- a/cores/arduino/delay.h
+++ b/cores/arduino/delay.h
@@ -60,40 +60,43 @@ extern void delay( uint32_t dwMs ) ;
*
* \param dwUs the number of microseconds to pause (uint32_t)
*/
-static inline void delayMicroseconds(uint32_t) __attribute__((always_inline, unused));
-static inline void delayMicroseconds(uint32_t usec){
- if (usec == 0) return;
- uint32_t n = usec * (VARIANT_MCK / 3000000);
-#if 0
- __asm__ volatile(
- "L_%=_delayMicroseconds:" "\n\t"
- "subs %0, %0, #1" "\n\t"
- "bne L_%=_delayMicroseconds" "\n"
- : "+r" (n) :
- );
-#else
- for ( ; n != 0 ; n-- )
+static __inline__ void delayMicroseconds( uint32_t ) __attribute__((always_inline, unused)) ;
+static __inline__ void delayMicroseconds( uint32_t ul_usec )
+{
+ if ( ul_usec == 0 )
{
+ return ;
}
-#endif
-}
-/*
-__attribute__((naked)) static void delay_loop(unsigned n)
-{
- __asm volatile ("1: subs r0, r0, #1");
- __asm volatile (" bne 1b");
- __asm volatile (" bx lr");
-}
+ uint32_t ul = ul_usec * (VARIANT_MCK / 3000000); // = ul_usec * 16 with VARIANT_MCK @ 48MHz
-void delay_microseconds(unsigned n)
-{
- // Bogus assumption:
- // Assume 8 cycles/iteration and running at 80MHz
- delay_loop(n * 10);
-}
+/* following 'for' loop will generate this:
+ * loop:
+ * subs r3, #1 // 1 Core cycle
+ * bne.n loop // 1 or 2 Core cycles, depends on pipeline break (50/50?)
+
+ for ( ; ul != 0 ; ul-- )
+ {
+ __asm__ volatile("");
+ }
*/
+/*
+ * __asm__ [__volatile__] ( AssemblerTemplate : [OutputOperands] [ : [InputOperands] [ : [Clobbers] ] ] )
+ *
+ * __asm__ documentation for GCC: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
+ * __volatile__ documentation for GCC: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
+ *
+ */
+ __asm__ __volatile__( "1: \n"
+ "sub %0, #1 \n" /* substract 1 from %0 (ul) */
+ "bne 1b \n"
+ : /* no outputs */
+ : "r" (ul) /* 'ul' variable is '%0', '+' means R/W constraint on this variable/register, 'r' means 'register' (versus 'm' for 'memory' */
+ : /* no clobber, ie nothing should be damaged by this asm code */
+ ) ;
+}
+
#ifdef __cplusplus
}
#endif
--
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