Hardware Reference
In-Depth Information
The program
pullup.c
, shown next, provides a simple utility to change the pull-up
resistor settings. The program listing for
gpio_io.c
is provided in the “Direct Register
Access” section. The source for
timed_wait.c
is found in the “Source Code” section in
Chapter 1 of
Experimenting with Raspberry Pi
(Apress, 2014).
After compiling, the following example changes the GPIO 7 pull-up to high and GPIO
8 to low:
$ ./pullup 7=low 8=high
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2
pullup.c : Change the pull−up resistor setting for GPIO pin
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4
5 #include <stdio.h>
6 #include <stdlib.h>
7 #include <fcntl.h>
8 #include <unistd.h>
9 #include <errno.h>
10 #include <setjmp.h>
11 #include <sys/mman.h>
12 #include <signal.h>
13
14 #include "gpio_io.c" /
GPIO routines
/
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15 #include "timed_wait.c" /
Delay
/
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16
17 /
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18
0x7E200094 GPPUD GPIO Pin Pull−up/down Enable
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19
0x7E200098 GPPUDCLK0 GPIO Pin Pull−up/down Enable Clock 0
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20
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21
22 #define GPIO_GPPUD
(ugpio+37)
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23 #define GPIO_GPPUDCLK0
(ugpio+38)
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24
25 static inline void
26 gpio_setpullup(int gpio, int pull) {
27 unsigned mask = 1 << gpio; /
GPIOs 0 to 31 only
/
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28 unsigned pmask = pull >= 0 ? ( 1 << !! pull) : 0;
29
30 GPIO_GPPUD = pmask; /
Select pull−up setting
/
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31 timed_wait (0, 500, 0);
32 GPIO_GPPUDCLK0 = mask; /
Set the GPIO of interest
/
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33 timed_wait (0, 500, 0);
34 GPIO_GPPUD = 0; /
Reset pmask
/
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35 timed_wait (0, 500, 0);
36 GPIO_GPPUDCLK0 = 0; /
Set the GPIO of interest
/
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37 timed_wait (0, 500, 0);
38 }
39
