PC/CP320 Physical Computing
Raspberry Pi SPI Analog-to-Digital Converter
Overview
Digital transmission of analog data, (such as sound and video),
requires analog input signals to be converted to
digital signals for processing. Once received, the signals
then have
to be converted to back analog.
These conversions are done by analog to digital (A/D) and
digital to
analog (D/A) converters, respectively.
Although you could construct
these converters from discrete components,
integrated circuits specifically designed for these purposes are
usually used.
Objectives
- To use the SPI interface on the Raspberry Pi
- To become familiar with analog-to-digital conversion
- To write a test program to show the SPI A/D in operation
- To introduce the infrared distance sensor
Background
The SPI (
Serial
Peripheral
Interface bus
is a
synchronous interface allowing a single
master device to communicate with multiple
slave devices. The master
device controls a clock signal for the slaves. The bus consists of
two data lines (one for each direction), the clock signal, and select
lines for each slave.
Although it may not always be
mentioned, it's also important that the GROUND lines for the
master and slaves be connted.
It is now very commonly used to connect sensors to
microprocessors or microcontrollers, and so will remain relevant for
the forseable future. It's also one of the simplest serial protocols,
and so it's easy to study.
The Raspberry Pi has an SPI bus built-in to handle up to two slave
devices, so it can be used for
SPI communications.
Preparation
Equipment
Procedure
|
The Rapsberry Pi GPIO pins can easily be damaged with
improper handling. The ADC in this lab can use a 3.3V
supply, so it can be powered from the 3.3V GPIO pin.
|
To wire the ADC, there are 4 different sub-circuits to wire.
They are:
-
Supply voltages and inputs from Raspberry Pi
-
Voltage reference
-
SPI connections to Raspberry Pi
-
Analog inputs
You will wire each of these sections in turn.
-
SPI GPIO pins
Note: The Spidev library has laready been installed.
From the GPIO diagram,
identify all of the SPI pins, namely:
- Connect VDD.
- Connect AGND and DGND.
- Conect the REF input of the ADC to the 3.3V pin of the
Raspberry Pi.
The input voltages will be converted relative to
the voltage on the REF input.
-
Most of the SPI pins should already be connected to the
MAX522 DAC. Daisy-chain the pins from there to the
ADC. There will be two other pins that will need to come
directly from the Pi.
Since communication with the Raspberry Pi
is bidirectional, which Raspberry Pi SPI signal
is needed which was not needed for the DAC?
Which SPI slave select line was already used for the DAC?
What changes will you need to make to your code to use the
other one with the ADC?
- Conect one analog input to the variable supply. Make sure
your grounds are connected as well.
- Write a program to read values from one input of the ADC.
Note that every
transfer to the ADC should be a multi-byte transfer.
Download the
test program from the website.
Replace the question marks with appropriate values as needed.
Note: there is a potential problem
with the
sample above if you are using a DAC
and an ADC
on the same SPI port but do NOT have them
as different devices; i.e. using
different chip select lines.
This would prevent them from being connected at the same time.
Configure the code if necessary so that this is not a
problem.
-
If you
have
done everything correctly, you should have a circuit which can
take in an
analog voltage
and display the value for an input
between 0 and 3.3 volts.
-
Once you have it working, replace the variable supply with the
infrared distance sensor.
-
Get rid of the magic numbers in the code sample.
(Hint: Look at the ADC datasheet.)
Demonstrate your circuit to the lab supervisor.
Keep this circuit for future labs.
