raspberry-gpio-python（树莓派GPIO与Python编程）

国外的设计接口设计得很棒，包括问题：读脏与防抖，还包括读这个数据提供了两种方式，一种是阻塞等待方式，还有一种是回调函数，前一种是通讯中常用的方式，后一种来自系统架构设计的整体性考虑。这种硬件接口设计的思路值得我们学习。

Inputs（输入）

There are several ways of getting GPIO input into your program. The first and simplest way is to check the input value at a point in time. This is known as 'polling' and can potentially miss an input if your program reads the value at the wrong time. Polling is performed in loops and can potentially be processor intensive. The other way of responding to a GPIO input is using 'interrupts' (edge detection). An edge is the name of a transition from HIGH to LOW (falling edge) or LOW to HIGH (rising edge).

中文速读：有很多方式将gpio输入我们的程序，最简单的是轮询，这种方式在不同时间采集数据会丢失，还多花费CPU资源，另一种方式为中断，中断依据边缘进行触发（数字电路中学过的）到么下边界触发，要么上边界触发。

Pull up / Pull down resistors（下拉、下拉电阻）

If you do not have the input pin connected to anything, it will 'float'. In other words, the value that is read in is undefined because it is not connected to anything until you press a button or switch. It will probably change value a lot as a result of receiving mains interference.

To get round this, we use a pull up or a pull down resistor. In this way, the default value of the input can be set. It is possible to have pull up/down resistors in hardware and using software. In hardware, a 10K resistor between the input channel and 3.3V (pull-up) or 0V (pull-down) is commonly used. The RPi.GPIO module allows you to configure the Broadcom SOC to do this in software:

中文速读：当接口没接任何设备时，这个值读出来可能有多种结果，这是“变动的”，解决它我们可以用下拉或下拉数阻，通常10k数阻。那么gpio通过如下形式配置：

GPIO.setup(channel, GPIO.IN, pull_up_down=GPIO.PUD_UP)
  # or
GPIO.setup(channel, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

(where channel is the channel number based on the numbering system you have specified - BOARD or BCM).

Testing inputs (polling)测试输入

You can take a snapshot of an input at a moment in time:

if GPIO.input(channel):
    print('Input was HIGH')
else:
    print('Input was LOW')

To wait for a button press by polling in a loop:

while GPIO.input(channel) == GPIO.LOW:
    time.sleep(0.01)  # wait 10 ms to give CPU chance to do other things慢0.01让cpu去做其它事，其它进程的并发性能考虑

(this assumes that pressing the button changes the input from LOW to HIGH)

中文速读：这里假设按键从低到高改变输入

Interrupts and Edge detection（中断与边界检测）

An edge is the change in state of an electrical signal from LOW to HIGH (rising edge) or from HIGH to LOW (falling edge). Quite often, we are more concerned by a change in state of an input than it's value. This change in state is an event.

To avoid missing a button press while your program is busy doing something else, there are two ways to get round this:

中文速读：改变边界状态是从电平信号从低到高，或者从高到低，与此同时，我们更关心的是输入的状态值，这种改变就是事件，为了避免并发丢失，即事件丢失，gpio提供了两种方式来解决忙中丢失问题：阻塞等待与线程回调函数

• the wait_for_edge() function
• the event_detected() function
• a threaded callback function that is run when an edge is detected

wait_for_edge() function（阻断意外，等边界检测）

The wait_for_edge() function is designed to block execution of your program until an edge is detected. In other words, the example above that waits for a button press could be rewritten as:

GPIO.wait_for_edge(channel, GPIO.RISING)

Note that you can detect edges of type GPIO.RISING, GPIO.FALLING or GPIO.BOTH. The advantage of doing it this way is that it uses a negligible amount of CPU, so there is plenty left for other tasks.

If you only want to wait for a certain length of time, you can use the timeout parameter:

# wait for up to 5 seconds for a rising edge (timeout is in milliseconds)
channel = GPIO.wait_for_edge(channel, GPIO_RISING, timeout=5000)
if channel is None:
    print('Timeout occurred')
else:
    print('Edge detected on channel', channel)
中文速读：这种方式是指阻塞了，等了多长时间可以在GPIO上设timeout参数，这种适用于异步工作方式，我们不清楚要等多少时间，因些应用场景对实时要求不高的。

event_detected() function

The event_detected() function is designed to be used in a loop with other things, but unlike polling it is not going to miss the change in state of an input while the CPU is busy working on other things. This could be useful when using something like Pygame or PyQt where there is a main loop listening and responding to GUI events in a timely basis.

GPIO.add_event_detect(channel, GPIO.RISING)  # add rising edge detection on a channel
do_something()
if GPIO.event_detected(channel):
    print('Button pressed')

Note that you can detect events for GPIO.RISING, GPIO.FALLING or GPIO.BOTH.

中文速读：主动测试

Threaded callbacks

RPi.GPIO runs a second thread for callback functions. This means that callback functions can be run at the same time as your main program, in immediate response to an edge. For example:

def my_callback(channel):
    print('This is a edge event callback function!')
    print('Edge detected on channel %s'%channel)
    print('This is run in a different thread to your main program')

GPIO.add_event_detect(channel, GPIO.RISING, callback=my_callback)  # add rising edge detection on a channel
...the rest of your program...

If you wanted more than one callback function:

def my_callback_one(channel):
    print('Callback one')

def my_callback_two(channel):
    print('Callback two')

GPIO.add_event_detect(channel, GPIO.RISING)
GPIO.add_event_callback(channel, my_callback_one)
GPIO.add_event_callback(channel, my_callback_two)

Note that in this case, the callback functions are run sequentially, not concurrently. This is because there is only one thread used for callbacks, in which every callback is run, in the order in which they have been defined.

中文速读：就是回调函数

Switch debounce（消抖）

You may notice that the callbacks are called more than once for each button press. This is as a result of what is known as 'switch bounce'. There are two ways of dealing with switch bounce:

• add a 0.1uF capacitor across your switch.
• software debouncing
• a combination of both

To debounce using software, add the bouncetime= parameter to a function where you specify a callback function. Bouncetime should be specified in milliseconds. For example:

# add rising edge detection on a channel, ignoring further edges for 200ms for switch bounce handling
GPIO.add_event_detect(channel, GPIO.RISING, callback=my_callback, bouncetime=200)

or

GPIO.add_event_callback(channel, my_callback, bouncetime=200)

Remove event detection（取消事件测试）

If for some reason, your program no longer wishes to detect edge events, it is possible to stop them:

GPIO.remove_event_detect(channel)