User Manual

use, then calls setup_gpio to choose which pins to use and how to use them. In setup_gpio, as

usual INP_GPIO(n) (where n is the pin number) is used to activate the pins. This also sets them up

to be used as inputs. They should however, be used as an SPI bus, which is one of the alternative

functions for these pins (it is alternate 0). Thus we use SET_GPIO_ALT(n, a) (where n is the pin

number and a is the alternate number, in this case 0) to select this alternate use of the pins. Then the

program sends different values to the D/A and asks for real verification, using the multimeter, that the

D/A converter is generating the correct output voltage.

atod

To test the A/D, the atod program first asks which channel should be used and prints out the

connections required on Gertboard to run the program. Then it calls setup_io to get the GPIO

ready, then calls setup_gpio to choose which pins will be used, and how they will be used. The

setup_gpio used in atod works the same way as the one in dtoa (except for activating GPIO8

instead of GPIO7).

Then atod repeatedly reads the 10 bit value from the A/D converter and prints out the value on the

terminal, both as an absolute number and as a bar graph (the value read is divided by 16, and the

quotient is represented as a string of „#‟ characters). One thing to be aware of is that even if the

potentiometer is not moved, exactly the same result may not appear on successive reads. With 10 bits

of accuracy, it is very sensitive, and even the smallest changes, such as house current running in

nearby wires, can affect the value read.

dad

To test both the D/A and A/D at the same time, the dad test sends 17 different digital values to the

D/A (0 to 255 in even jumps, then back down to 0). The resulting values are then read in from the

A/D. Both the original digital values sent and the values read back are printed out, as is a bar graph

representing the value read back (divided by 16 as in atod). The bar graph printed out should be a

triangle shape: the lines will start out very short, then get longer and longer as larger digital values are

read back, then will get shorter again.

D/A and A/D tests in Python

The analogue to digital and digital to analogue converters are connected to the (SPI) ports on the

Raspberry Pi. These are the alternative functions of GPIO ports 7 and 8.

In order to make use of atod.py, dtoa.py and dad.py you must have SPI enabled, and you must

install a Python module called py-spidev. Instructions on how to do this are in the README.txt

included with the Python programs that you downloaded.

atod.py

The user chooses which channel to use on the analogue to digital converter (A/D), then wiring

instructions are printed out. The function get_adc(channel) uses spidev to read the A/D. It

receives three bytes of data and extracts the result – a number between 0 and 1023, where 0 means 0V

and 1023 means 3.3V.

The main loop runs 600 times with a “sleep” of 0.05s, so the program takes about 30 seconds (600 *

0.05) to run. During each iteration of the loop, the program reads the A/D and displays this reading

and a number of # symbols proportional to the value read, which depends on the position of the