树莓派 GPIO python

]raspberry-gpio-python example wiki

 (2014-10-02 09:56:49)


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标签：  树莓派   rpi.gpio   python

以下内容转载自：

http://sourceforge.net/p/raspberry-gpio-python/wiki/Examples/

[code]===========================================================

RPi.GPIO module basics

Importing the module

To import the RPi.GPIO module:

import RPi.GPIO as GPIO

By doing it this way, you can refer to it as just GPIO through the rest of your script.

To import the module and check to see if it is successful:

try:
import RPi.GPIO as GPIOexcept RuntimeError:
print("Error importing RPi.GPIO!  This is probably because you need superuser privileges.  You can achieve this by using 'sudo' to run your script")

Pin numbering

There are two ways of numbering the IO pins on a Raspberry Pi within RPi.GPIO. The first is using the BOARD numbering system. This refers to the pin numbers on the P1 header of the Raspberry Pi board. The advantage of using this numbering system is that your
hardware will always work, regardless of the board revision of the RPi. You will not need to rewire your connector or change your code.

The second numbering system is the BCM numbers. This is a lower level way of working - it refers to the channel numbers on the Broadcom SOC. You have to always work with a diagram of which channel number goes to which pin on the RPi board. Your script could
break between revisions of Raspberry Pi boards.

To specify which you are using using (mandatory):

GPIO.setmode(GPIO.BOARD)
# or
GPIO.setmode(GPIO.BCM)

Warnings

It is possible that you have more than one script/circuit on the GPIO of your Raspberry Pi. As a result of this, if RPi.GPIO detects that a pin has been configured to something other than the default (input), you get a warning when you try to configure a script.
To disable these warnings:

GPIO.setwarnings(False)

Setup up a channel

You need to set up every channel you are using as an input or an output. To configure a channel as an input:

GPIO.setup(channel, GPIO.IN)

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

More advanced information about setting up input channels can be found here.

To set up a channel as an output:

GPIO.setup(channel, GPIO.OUT)

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

You can also specify an initial value for your output channel:

GPIO.setup(channel, GPIO.OUT, initial=GPIO.HIGH)

Input

To read the value of a GPIO pin:

GPIO.input(channel)

(where channel is the channel number based on the numbering system you have specified (BOARD or BCM)). This will return either 0 / GPIO.LOW / False or 1 / GPIO.HIGH / True.

Output

To set the output state of a GPIO pin:

GPIO.output(channel, state)

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

State can be 0 / GPIO.LOW / False or 1 / GPIO.HIGH / True.

Cleanup

At the end any program, it is good practice to clean up any resources you might have used. This is no different with RPi.GPIO. By returning all channels you have used back to inputs with no pull up/down, you can avoid accidental damage to your RPi by shorting
out the pins. Note that this will only clean up GPIO channels that your script has used.

To clean up at the end of your script:

GPIO.cleanup()

It is possible that you only want to clean up one channel, leaving some set up when your program exits:

GPIO.cleanup(channel)

RPi Board Revision and RPi.GPIO version

To discover the Raspberry Pi board revision:

GPIO.RPI_REVISION

To discover the version of RPi.GPIO:

GPIO.VERSION

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Inputs

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).

Pull up / Pull down resistors

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:

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)

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

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

Interrupts and Edge detection

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:

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

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.

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.

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.

Switch debounce

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

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

GPIO.remove_event_detect(channel)

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GPIO Outputs

1. First set up RPi.GPIO (as described here)

import RPi.GPIO as GPIOGPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.OUT)

2. To set an output high:

GPIO.output(12, GPIO.HIGH)
# or
GPIO.output(12, 1)
# or
GPIO.output(12, True)

3. To set an output low:

GPIO.output(12, GPIO.LOW)
# or
GPIO.output(12, 0)
# or
GPIO.output(12, False)

4. Clean up at the end of your program

GPIO.cleanup()

Note that you can read the current state of a channel set up as an output using the input() function. For example to toggle an output:

GPIO.output(12, not GPIO.input(12))

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Using PWM in RPi.GPIO

To create a PWM instance:

p = GPIO.PWM(channel, frequency)

To start PWM:

p.start(dc)   # where dc is the duty cycle (0.0 <= dc <= 100.0)

To change the frequency:

p.ChangeFrequency(freq)   # where freq is the new frequency in Hz

To change the duty cycle:

p.ChangeDutyCycle(dc)  # where 0.0 <= dc <= 100.0

To stop PWM:

p.stop()

Note that PWM will also stop if the instance variable 'p' goes out of scope.
An example to blink an LED once every two seconds:

import RPi.GPIO as GPIOGPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.OUT)

p = GPIO.PWM(12, 0.5)
p.start(1)
input('Press return to stop:')   # use raw_input for Python 2
p.stop()GPIO.cleanup()

An example to brighten/dim an LED:

import time
import RPi.GPIO as GPIOGPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.OUT)

p = GPIO.PWM(12, 50)  # channel=12 frequency=50Hz
p.start(0)
try:
while 1:
for dc in range(0, 101, 5):
p.ChangeDutyCycle(dc)
time.sleep(0.1)
for dc in range(100, -1, -5):
p.ChangeDutyCycle(dc)
time.sleep(0.1)
except KeyboardInterrupt:
pass
p.stop()GPIO.cleanup()

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gpio_function(channel)

Shows the function of a GPIO channel.For example:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BOARD)
func = GPIO.gpio_function(pin)

will return a value from:

GPIO.INPUT, GPIO.OUTPUT, GPIO.SPI, GPIO.I2C, GPIO.HARD_PWM, GPIO.SERIAL, GPIO.UNKNOWN