48
50
Obviously, in that case, no \c \#define statement should be
51
The functions in this header file implement simple delay loops
52
that perform a busy-waiting. They are typically used to
53
facilitate short delays in the program execution. They are
54
implemented as count-down loops with a well-known CPU cycle
55
count per loop iteration. As such, no other processing can
56
occur simultaneously. It should be kept in mind that the
57
functions described here do not disable interrupts.
59
In general, for long delays, the use of hardware timers is
60
much preferrable, as they free the CPU, and allow for
61
concurrent processing of other events while the timer is
62
running. However, in particular for very short delays, the
63
overhead of setting up a hardware timer is too much compared
64
to the overall delay time.
66
Two inline functions are provided for the actual delay algorithms.
68
Two wrapper functions allow the specification of microsecond, and
53
The functions in this header file are wrappers around the basic
54
busy-wait functions from <util/delay_basic.h>. They are meant as
55
convenience functions where actual time values can be specified
56
rather than a number of cycles to wait for. The idea behind is
57
that compile-time constant expressions will be eliminated by
58
compiler optimization so floating-point expressions can be used
59
to calculate the number of delay cycles needed based on the CPU
60
frequency passed by the macro F_CPU.
62
\note In order for these functions to work as intended, compiler
63
optimizations <em>must</em> be enabled, and the delay time
64
<em>must</em> be an expression that is a known constant at
65
compile-time. If these requirements are not met, the resulting
66
delay will be much longer (and basically unpredictable), and
67
applications that otherwise do not use floating-point calculations
68
will experience severe code bloat by the floating-point library
69
routines linked into the application.
71
The functions available allow the specification of microsecond, and
69
72
millisecond delays directly, using the application-supplied macro
70
F_CPU as the CPU clock frequency (in Hertz). These functions
71
operate on double typed arguments, however when optimization is
72
turned on, the entire floating-point calculation will be done at
73
F_CPU as the CPU clock frequency (in Hertz).
75
\note When using _delay_us() and _delay_ms(), the expressions
76
passed as arguments to these functions shall be compile-time
77
constants, otherwise the floating-point calculations to setup the
78
loops will be done at run-time, thereby drastically increasing
79
both the resulting code size, as well as the time required to
83
77
#if !defined(__DOXYGEN__)
84
static inline void _delay_loop_1(uint8_t __count) __attribute__((always_inline));
85
static inline void _delay_loop_2(uint16_t __count) __attribute__((always_inline));
86
78
static inline void _delay_us(double __us) __attribute__((always_inline));
87
79
static inline void _delay_ms(double __ms) __attribute__((always_inline));
90
/** \ingroup util_delay
92
Delay loop using an 8-bit counter \c __count, so up to 256
93
iterations are possible. (The value 256 would have to be passed
94
as 0.) The loop executes three CPU cycles per iteration, not
95
including the overhead the compiler needs to setup the counter
98
Thus, at a CPU speed of 1 MHz, delays of up to 768 microseconds
102
_delay_loop_1(uint8_t __count)
112
/** \ingroup util_delay
114
Delay loop using a 16-bit counter \c __count, so up to 65536
115
iterations are possible. (The value 65536 would have to be
116
passed as 0.) The loop executes four CPU cycles per iteration,
117
not including the overhead the compiler requires to setup the
118
counter register pair.
120
Thus, at a CPU speed of 1 MHz, delays of up to about 262.1
121
milliseconds can be achieved.
124
_delay_loop_2(uint16_t __count)
127
"1: sbiw %0,1" "\n\t"
135
83
/* prevent compiler error by supplying a default */
136
84
# warning "F_CPU not defined for <util/delay.h>"
137
85
# define F_CPU 1000000UL
89
# warning "Compiler optimizations disabled; functions from <util/delay.h> won't work as designed"
141
93
\ingroup util_delay
added
removed
include/util/delay.h
