HW Expander design

Overview

In the previous phase of the development cycle we have designed and manufactured so called capes for the BeagleBone Black devices for the following purposes:

• We wanted to power the units from 12VDC source instead of 5VDC

• Better access for I2C interface

• We had needs for easy-to-use ports to attach additional circuits to the main board

We call them expanders, and after the cape installment these expanders could be use to extend the functionality of the BeagleBone unit.

Figure 1. BeagleBone Black cape

Figure 2. BeagleBone Black cape

Expander design

There are 8 general purpose expander slots on each cape, therefore each slot has the following pinout (front view).

Figure 3. Expander pin layout

Table 1. Expander pin layout

pin3	pin2	pin1	pin0
3V3	5V	Gnd	12V

The upper row of each connector is dedicated for GPIO connections. Two of the GPIO pins connected to the application processor, and two of the GPIO pins are connected to the PRU unit. The pins connected to the application processor are labeled G0..G15, and the pins connected to the PRUs are called P0..P15.

With this setup, the PRU and the application processor can cooperate on hardware level. Also, if we want we could use the P0..P15 GPIOs from the application processor as well.

Before applying input on these pins, you must ensure that they are configured as inputs and the highest voltage level is not higher than 3.3VDC. Higher voltages on these pins could destroy the BeagleBone unit.

GPIO basic usage

On the BeagleBone systems we can use the sysfs interface, where we can set the direction of the GPIOs, set or read value of the pins, and other several features as well. In order to use this feature, we need to know the ID of each pin, which are enlisted in the following table.

Table 2. BeagleBone pin-sysfs mapping

Expander	GPIO name	GPIO number
Expander 1	P0	86
	P1	88
	G0	67
	G1	66
Expander 2	P2	87
	P3	89
	G2	69
	G3	68
Expander 3	P4	76
	P5	77
	G4	11
	G5	10
Expander 4	P6	70
	P7	71
	G6	79
	G7	78
Expander 5	P8	20
	P9	7
	G8	2
	G9	3
Expander 6	P10	110
	P11	112
	G10	51
	G11	48
Expander 7	P12	111
	P13	113
	G12	31
	G13	50
Expander 8	P14	115
	P15	117
	G14	60
	G15	30

With the following script, you can set up a GPIO as an output, and set its value LOW or HIGH. All GPIO need to be exported before its usage, and if it is no longer needed, it must be unexported. These nodes of the file-system can only be written by root.

Basic GPIO usage with sysfs commands

echo 30 > /sys/class/gpio/export
echo "out" > /sys/class/gpio/gpio30/direction
echo "0" > /sys/class/gpio/gpio30/value
echo "1" > /sys/class/gpio/gpio30/value
echo 30 > /sys/class/gpio/unexport