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+定时器设备
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+===
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+
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+##功能
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+---
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+* 时间测量
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+* 周期或单次执行回调函数
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+
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+##编译
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+---
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+1. 在rtconfig.h添加 `#define RT_USING_HWTIMER`
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+
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+##使用流程
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+---
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+1. 以读写方式打开设备
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+2. 设置超时回调函数（如果需要）
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+3. 根据需要设置定时模式（单次/周期）
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+4. 设置计数频率（可选）
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+5. 写入超时值，定时器随即启动
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+6. 停止定时器（可选）
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+7. 关闭设备（如果需要）
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+
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+应用参考 [hwtimer_test] (/examples/test/hwtimer\_test.c)
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+
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+##驱动编写指南
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+---
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+###操作接口
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+
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+```
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+struct rt_hwtimer_ops
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+{
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+ void (*init)(struct rt_hwtimer_device *timer, rt_uint32_t state);
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+ rt_err_t (*start)(struct rt_hwtimer_device *timer, rt_uint32_t cnt, rt_hwtimer_mode_t mode);
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+ void (*stop)(struct rt_hwtimer_device *timer);
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+ rt_uint32_t (*count_get)(struct rt_hwtimer_device *timer);
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+ rt_err_t (*control)(struct rt_hwtimer_device *timer, rt_uint32_t cmd, void *args);
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+};
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+```
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+
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+* init - state <1 打开设备 0 关闭设备>
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+* start - cnt <超时值> - mode <单次/周期>
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+* stop - <停止计数>
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+* count_get - <读取计数器值>
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+* control - <设置计数频率 >
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+
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+###定时器特征信息
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+
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+```
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+struct rt_hwtimer_info
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+{
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+ rt_int32_t maxfreq;
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+ rt_int32_t minfreq;
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+ rt_uint32_t maxcnt;
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+ rt_uint8_t cntmode;
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+ };
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+```
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+
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+* maxfreq <设备支持的最大计数频率>
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+* minfreq <设备支持的最小计数频率>
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+* maxcnt <计数器最大计数值>
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+* cntmode <递增计数/递减计数>
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+
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+###注册设备
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+```
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+static rt_hwtimer_t _timer0;
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+int stm32_hwtimer_init(void)
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+{
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+ _timer0.info = &_info;
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+ _timer0.ops = &_ops;
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+
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+ rt_device_hwtimer_register(&_timer0, "timer0", TIM2);
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+
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+ return 0;
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+}
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+```
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+
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+###定时器中断
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+```
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+void timer_irq_handler(void)
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+{
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+ //其它操作
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+
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+ rt_device_hwtimer_isr(&_timer0);
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+}
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+```
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+
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+##注意事项
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+---
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+
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+<font color="#FF0000">可能出现定时误差</font>
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+
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+
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+误差原因：
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+
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+假设计数器最大值0xFFFF，计数频率1Mhz，定时时间1秒又1微秒。
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+
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+由于定时器一次最多只能计时到65535us，对于1000001us的定时要求
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+就需分15次完成。误差时间: 1000001 - (65535*15) = 16976us。
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+这个误差太大，为了减小误差最理想的情况就是整除。实际中不可能
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+所有数都被整除(1不合适)，所以就只有最大限度逼近理想情况。例如
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+上式中以 50000us定时20次即可将误差减小到1us(理论值,忽略指令时间)
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heyuanjie87