我正在尝试实施Dallas OneWire协议，但我在STM32l-Discovery上产生微秒延迟时遇到了麻烦。
如何实现足够准确的定时器以将程序延迟x微秒？
答案 0 :(得分：3)
首先，我必须告诉你，使用软件无法实现精确的usec延迟。即使您使用基于中断的系统，也会有延迟。当然，您可以通过更大的CPU频率实现更高的精度。
为了连接1-Wire器件，您可以使用：
对于第二种解决方案，您必须调用基于软件的延迟。您可以进行标志轮询延迟或基于中断的标志轮询延迟。在这两种情况下，都会确保已经过了一定的时间，但您无法确定更多的时间已经过去了。这是因为CPU延迟，CPU时钟等...
例如，考虑以下实现。我们对HW TIMER进行编程以连续计数，并检查TIMER的值。我们将每个TIMER的刻度和jiffies之间的时间命名为“jiffy”TIMERS最大值：
低级驱动程序部分（例如：driver.h）
// ...
#define JF_TIM_VALUE (TIM7->CNT)
int JF_setfreq (uint32_t jf_freq, uint32_t jiffies);
// ...
低级别驱动程序部分（例如：driver.c）
// ...
#include <stm32l1xx.h>
#include <misc.h>
#include <stm32l1xx_rcc.h>
#include <stm32l1xx_tim.h>
/*
* Time base configuration using the TIM7
* \param jf_freq The TIMER's frequency
* \param jiffies The TIMER's max count value
*/
int JF_setfreq (uint32_t jf_freq, uint32_t jiffies) {
uint32_t psc=0;
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM7, ENABLE);
SystemCoreClockUpdate ();
if (jf_freq)
psc = (SystemCoreClock / jf_freq) - 1;
if (psc < 0xFFFF) TIM7->PSC = psc;
else return 1;
if (jiffies < 0xFFFF) TIM7->ARR = jiffies;
else return 1;
TIM7->CR1 |= TIM_CR1_CEN;
return 0;
}
// ...
具有一些延迟实现的中间件jiffy系统 的 jiffy.h：
#include "string.h"
typedef int32_t jiffy_t; // Jiffy type 4 byte integer
typedef int (*jf_setfreq_pt) (uint32_t, uint32_t); //Pointer to setfreq function
typedef volatile struct {
jf_setfreq_pt setfreq; // Pointer to driver's timer set freq function
jiffy_t *value; // Pointer to timers current value
uint32_t freq; // timer's frequency
uint32_t jiffies; // jiffies max value (timer's max value)
jiffy_t jpus; // Variable for the delay function
}jf_t;
/*
* ============= PUBLIC jiffy API =============
*/
/*
* Link functions
*/
void jf_link_setfreq (jf_setfreq_pt pfun);
void jf_link_value (jiffy_t* v);
/*
* User Functions
*/
void jf_deinit (void);
int jf_init (uint32_t jf_freq, uint32_t jiffies);
jiffy_t jf_per_usec (void);
void jf_delay_us (int32_t usec);
int jf_check_usec (int32_t usec);
<强> jiffy.c：
#include "jiffy.h"
static jf_t _jf;
#define JF_MAX_TIM_VALUE (0xFFFF) // 16bit counters
//Connect the Driver's Set frequency function
void jf_link_setfreq (jf_setfreq_pt pfun) {
_jf.setfreq = pfun;
}
// Connect the timer's value to jiffy struct
void jf_link_value (jiffy_t* v) {
_jf.value = v;
}
// De-Initialize the jf data and un-connect the functions
// from the driver
void jf_deinit (void) {
memset ((void*)&_jf, 0, sizeof (jf_t));
}
// Initialise the jf to a desired jiffy frequency f
int jf_init (uint32_t jf_freq, uint32_t jiffies) {
if (_jf.setfreq) {
if ( _jf.setfreq (jf_freq, jiffies) )
return 1;
_jf.jiffies = jiffies;
_jf.freq = jf_freq;
_jf.jpus = jf_per_usec ();
return 0;
}
return 1;
}
// Return the systems best approximation for jiffies per usec
jiffy_t jf_per_usec (void) {
jiffy_t jf = _jf.freq / 1000000;
if (jf <= _jf.jiffies)
return jf;
else
// We can not count beyond timer's reload
return 0;
}
/*!
* \brief
* A code based delay implementation, using jiffies for timing.
* This is NOT accurate but it ensures that the time passed is always
* more than the requested value.
* The delay values are multiplications of 1 usec.
* \param
* usec Time in usec for delay
*/
void jf_delay_us (int32_t usec) {
jiffy_t m, m2, m1 = *_jf.value;
usec *= _jf.jpus;
if (*_jf.value - m1 > usec) // Very small delays will return here.
return;
// Delay loop: Eat the time difference from usec value.
while (usec>0) {
m2 = *_jf.value;
m = m2 - m1;
usec -= (m>0) ? m : _jf.jiffies + m;
m1 = m2;
}
}
/*!
* \brief
* A code based polling version delay implementation, using jiffies for timing.
* This is NOT accurate but it ensures that the time passed is always
* more than the requested value.
* The delay values are multiplications of 1 usec.
* \param
* usec Time in usec for delay
*/
int jf_check_usec (int32_t usec) {
static jiffy_t m1=-1, cnt;
jiffy_t m, m2;
if (m1 == -1) {
m1 = *_jf.value;
cnt = _jf.jpus * usec;
}
if (cnt>0) {
m2 = *_jf.value;
m = m2-m1;
cnt-= (m>0) ? m : _jf.jiffies + m;
m1 = m2;
return 1; // wait
}
else {
m1 = -1;
return 0; // do not wait any more
}
}
嗯，你直到这里。尼斯
所以现在你可以在你的应用程序中使用它，如下所示： 的 main.c中：
#include "driver.h"
#include "jiffy.h"
void do_some_job1 (void) {
// job 1
}
void do_some_job2 (void) {
// job 2
}
int main (void) {
jf_link_setfreq ((jf_setfreq_pt)JF_setfreq); // link with driver
jf_link_value ((jiffy_t*)&JF_TIM_VALUE);
jf_init (1000000, 1000); // 1MHz timer, 1000 counts, 1 usec per count
// use delay version
do_some_job1 ();
jf_delay_us (300); // wait for at least 300 usec
do_some_job1 ();
// use polling version
do_some_job1 ();
while (jf_check_usec (300)) {
do_some_job2 (); // keep calling for at least 300 usec
}
}