Real-Time I/O - BeagleBone - page 532

Figure 8-2. Diagram for wiring a pushbutton and LED with the LED attached to P9_13
Now, flash the LED attached to P9_13 using the Linux sysfs interface ( Recipe 7.4 ). To
do this, first look up which GPIO number P9_13 is attached to by referring to Fig-
ure 5-18 . Finding P9_13 at GPIO 31, export GPIO 31 and make it an output:
bone# cd cd /sys/class/gpio/
bone# echo 31 > export
bone# cd gpio31
bone# echo out > direction
bone# echo 1 > value
bone# echo 0 > value
The LED will turn on when 1 is echoed into value and off when 0 is echoed.
Now that you know the LED is working, look up its memory address. This is where things
get very detailed. First, download the AM335x Technical Reference Manual . Look up
GPIO0 in the Memory Map chapter (sensors). Table 2-2 indicates that GPIO0 starts at
address 0x44E0_7000 . Then go to Section 25.4.1, “GPIO Registers.” This shows that
GPIO_DATAIN has an offset of 0x138 , GPIO_CLEARDATAOUT has an offset of
0x190 , and GPIO_SETDATAOUT has an offset of 0x194 .
This means you read from address 0x44E0_7000 + 0x138 = 0x44E0_7138 to see
the status of the LED:
bone# devmem2 0x44E07138
/dev/mem opened.
Memory mapped at address 0xb6f8e000.