More Raspberry Pi Electronics Experiments – Gertboard and Potentiometer-Controlled LED Cluster

One of the things which alerted me to the potential of the Raspberry Pi as an electronics control system was the announcement of the Gertboard before the Raspberry Pi was released into the market. When the Gertboard was announced for sale in November 2012, it was fully my intention to buy one, but a lack of money kept me from purchasing it at that point. The unassembled Gertboard kits soon sold out, leaving element14, who distribute the Gertboard, to decide to release an assembled Gertboard. This went on sale very recently, and shortly after release, I bought one from Premier Farnell’s Irish subsidiary.

The Gertboard, for the uninitiated, is an I/O expansion board that plugs into the GPIO header on the Raspberry Pi. Designed by Gert van Loo, designer of the Raspberry Pi alpha hardware, the Gertboard not only protects the GPIO pins from physical and electrical damage, but also provides a set of additional features. These include input/output buffers, a motor controller, an open collector driver, an MCP3002 ADC and MCP4802 DAC and an Atmel ATMega328P microcontroller which is compatible with the Arduino IDE and programming environment.

I was very impressed by the quick response from element14 after my purchase; my delivery came only two days after ordering and would have come even sooner if I hadn’t missed the 20.00 deadline on the day I had ordered it. The Gertboard was packaged with a number of female-to-female jumper wires, a set of jumpers, plastic feet for the board and a CD-ROM with a set of development tools for the ARM Cortex-M platform.

So far, I’ve only had occasion to test the buffered I/O, the ADC and DAC and the microcontroller; I still don’t have parts to test the motor controller or open collector driver. Aside from some documented peculiarities regarding the input buffers when at a floating voltage, including the so-called “proximity sensor” effect, things seem to have been going rather well.

The acquisition of the Gertboard gave me the impetus to really get down to trying to test my own expansions to the simple test circuits I had implemented before. One interesting application that I considered was to use a potentiometer to control a bank of LEDs in order to provide some sort of status indication.

The following Fritzing circuit diagram shows the layout of this circuit without the use of the Gertboard; the onboard LEDs and GPIO pins lined up in a row on the Gertboard makes it slightly less messy in terms of wiring.

In this diagram, GPIO pins 0, 1, 4, 17, 18, 21, 22 and 23 are used to control the LEDs, although you could also use pins 24 or 25 without conflict with either the SPI bus – which is necessary for the MCP3002 ADC – or the serial UART on pins 14 and 15. However, this is a lot of GPIO pins taken up for one application, which may warrant the use of a shift register or an I2C I/O expander such as the MCP23008 or MCP23017 in order to control more LEDs with less pins.

In order to control this circuit, I took the sample Gertboard test software and modified it slightly. As the potentiometer is turned to the right, the ADC value increases to a maximum of 1023; therefore, the distance between each LED’s activation point should be 1023 divided by 8 – very close to 128. The LEDs will light from left-to-right as the potentiometer’s resistance decreases, with one LED lighting at an ADC reading of 0, two LEDs at 128, all the way up to all eight LEDs at 1023.

//
// Gertboard Demo
//
// SPI (ADC/DAC) control code
//
// This code is part of the Gertboard test suite
//
//
// Copyright (C) Gert Jan van Loo & Myra VanInwegen 2012
// No rights reserved
// You may treat this program as if it was in the public domain
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
// POSSIBILITY OF SUCH DAMAGE.
//

#include "gb_common.h"
#include "gb_spi.h"

void setup_gpio(void);

int leds[] = {1 << 23, 1 << 22, 1 << 21, 1 << 18, 1 << 17, 1 << 4, 1 << 1,
	      1 << 0};

int main(void)
{
    int r, v, s, i, chan, nleds;

    do {
	printf("Which channel do you want to test? Type 0 or 1.\n");
	chan = (int) getchar();
	(void) getchar();
    } while (chan != '0' && chan != '1');

    printf("When ready, press Enter.");
    (void) getchar();

    setup_io();
    setup_gpio();
    setup_spi();

    for (r = 0; r < 1000000; r++) {
	v = read_adc(chan);
	for (i = 0; i < 8; i++) {
	    GPIO_CLR0 = leds[i];
	}
	nleds = v / (1023 / 8); /* number of LEDs to turn on */
	for (i = 0; i < nleds; i++) {
	    GPIO_SET0 = leds[i];
	}
	short_wait();
    }

    printf("\n");
    restore_io();
    return 0;
}

void setup_gpio()
{
    /* Setup alternate functions of SPI bus pins and SPI chip select A */
    INP_GPIO(8); SET_GPIO_ALT(8, 0);
    INP_GPIO(9); SET_GPIO_ALT(9, 0);
    INP_GPIO(10); SET_GPIO_ALT(10, 0);
    INP_GPIO(11); SET_GPIO_ALT(11, 0);
    /* Setup LED GPIO pins */
    INP_GPIO(23); OUT_GPIO(23);
    INP_GPIO(22); OUT_GPIO(22);
    INP_GPIO(21); OUT_GPIO(21);
    INP_GPIO(18); OUT_GPIO(18);
    INP_GPIO(17); OUT_GPIO(17);
    INP_GPIO(4); OUT_GPIO(4);
    INP_GPIO(1); OUT_GPIO(1);
    INP_GPIO(0); OUT_GPIO(0);
}

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