when you put a pin to mode INPUT. you can set pull up/down registers.
because when you booted raspberry pi ,all gpio’s have default PULL UP or PULL DOWN registers. previus article sample is working because when we put pin to INPUT, it’is LOW. if you use another pin that is HIGH it will not work.
look at below picture. it is taken from bcm2835 sheet page 102
now changing pull up/down
in document page 91, I have taken this address’s
0x 7E20 0094 GPIO Pin Pull-up/down Enable 32
0x 7E20 0098 GPIO Pin Pull-up/down Enable Clock 0
0x 7E20 009C GPPUDCLK1 GPIO Pin Pull-up/down Enable Clock 1
#define PERIPHERALS_GPIO_PULL_PHYSICAL (0x94/sizeof(int))
#define PERIPHERALS_GPIO_PULL_CLOCK_PHYSICAL (0x98/sizeof(int))
as you see we defined two address, why ?
because we will set PULL value to PERIPHERALS_GPIO_PULL_PHYSICAL address
and pin value to PERIPHERALS_GPIO_PULL_CLOCK_PHYSICAL address.
but procedure of changing pull value is more than this.
1. Write to GPPUD to set the required control signal (i.e. Pull-up or Pull-Down or neither
to remove the current Pull-up/down)
2. Wait 150 cycles – this provides the required set-up time for the control signal
3. Write to GPPUDCLK0/1 to clock the control signal into the GPIO pads you wish to
modify – NOTE only the pads which receive a clock will be modified, all others will
retain their previous state.
4. Wait 150 cycles – this provides the required hold time for the control signal
5. Write to GPPUD to remove the control signal
6. Write to GPPUDCLK0/1 to remove the clock
write correct bit values to (gpio+PERIPHERALS_GPIO_PULL_PHYSICAL)
00 = Off – disable pull-up/down
01 = Enable Pull Down control
10 = Enable Pull Up control
wait 5 microseconds
set correct pin bit to 1 at (gpio+PERIPHERALS_GPIO_PULL_CLOCK_PHYSICAL) adress
wait 5 microsecond
write zero to (gpio+PERIPHERALS_GPIO_PULL_PHYSICAL) address
wait 5 microsecond
write (gpio+PERIPHERALS_GPIO_PULL_CLOCK_PHYSICAL) adress
below is the algorithm.
now create a template board above. and test it
2 cables (yellow is output, and orange is input)
1 10K resistor
1 push button
as you see, orange cable is at 7 pin, which is HIGH when rebooted raspberry.
if you disable
code it will now work as expected.
reboot your raspberry before testing. when you change a pin PULL_DOWN, until you change again it will be PULL_DOWN.
here is test code pull.c
gcc -o test pull.c