User Manual

In the diagram in Figure 12, black circles show which pins are being connected, and black lines

between two pins indicate that jumpers (if they are adjacent) or straps (if they are further apart) are

used to connect them.

Buttons Test in C

The code specific to the buttons test is buttons.c. In the main routine, the connections

required for this test are firstly printed to the terminal (a text description of the wiring diagram above).

When the user verifies that the connections are correct, setup_io (described on page 12) is called

to get everything ready.

setup_gpio is then called, which gets GPIO pins 1 to 3 ready to be used as pushbutton inputs. It

does this by first using the macro INP_GPIO(n) (where n is the GPIO pin number) to select these 3

pins for input.

Then pins are required to be pulled high: the buttons work by dropping the voltage down to 0V when

the button is pressed, so it needs to be high when the button is not pressed. This is done by setting

GPIO_PULL to 2, the code for pull-up. Should it ever be required, the code for pull-down is 1. The

code for no pull is 0; this will allows this pin to be used for output after it has been used as a

pushbutton input. To apply this code to the desired pins, set GPIO_PULLCCLK0 = 0X03800000.

This hexadecimal number has bits 23, 24, and 25 set to 1 and all the rest set to 0. This means that the

pull code is applied to GPIO pins 23, 24, and 25. A short_wait allows time for this to take effect,

and then GPIO_PULL and GPIO_PULLCLK0 are set back to 0.

Back in the main routine, a loop is entered in which the button states are read (using macro

GPIO_IN0), grabbing bits 23, 24, and 25 using a shift and mask logical operations, and, if the button

state is different from before, it is printed out in binary: up (high) is printed as „1‟ and down (low) is

printed as „0‟. This loop executes until a sufficient number of button state changes have occurred.

After the loop, unpull_pins is called, which undoes the pull-up on the pins, then call

restore_io in gb_common.c to clean up.

Buttons Test in Python

This program (buttons-rg.py) is only available using the RPi.GPIO package at the moment

because the pull-up facility in WiringPi for Python is not yet available. The buttons cannot be used

without this facility.

First the program imports the RPi.GPIO module it needs to handle GPIO control. Then the command

GPIO.setmode(GPIO.BCM)sets up the BCM numbering scheme for the pins. The result of this is

that the pin numbers in the Python code are the numbers that the BCM2835 (the Raspberry Pi

processor) uses to refer to the pins. Otherwise, the numbers in the Python code will refer to the pin

numbers in the J1 header (which is the same as their placement in the P1 header on the RPi).

The next two lines set up ports 23-25 with pull-up:

for i in range(23,26):

GPIO.setup(i, GPIO.IN, pull_up_down=GPIO.PUD_UP)