2 Digit LED
2015 Adrian Krag
Objective
This document explores using the MCP23017, the port expander to create a 2 digit LED display.
An LED character is different from an LCD. The LCD has a built in controller that can write
both characters and letters. Sometimes that's more than is needed. If the project calls for only a
simple 2 digit number this approach can be very inexpensive and very easy to implement.
A 7 Segment digit requires 8 digital outputs (7 segments and a decimal point). Two digits would
require more pins than are available on the standard Arduino. To solve this issue, the
MCP23017 is a very useful device. This expander chip has 16 digital pins in 2 - 8 bit ports, one
to control each digit.
Writing Digits to the 7 Segment LED
The LED has 10 pins, 8 for the elements and 2 ground pins. Each element pin can turn on one
segment. To write a number to the display, the program writes one byte, 8 bits. If the individual
bit is a 1, the segment is on. 0 turns it off. The segments are wired to the port pins. Pins 21 to
28, General Purpose Port A are connected to the right digit. Pins 0 to 8 are connected to the left
digit. To create a digit the program just has to sets and clears the appropriate bits. Careful
analysis of the circuit will tell which bit in the Expander port activates which segment. However,
a little experimentation with different words will also give us the information.
bit
0
1
2
3
4
5
6
7
Seg
g
f
a
b
o
c
d
e
The segment on our board is set as shown in these
tables. To Write numbers use the segment, use the
following binary numbers. For example to turn on a 0
- segment f, a, b, c, d, e
To Create a digit
select the segments to turn on, and set the
corresponding bits in the port corresponding with the correct bits.
11101110B
00101000B
11001101B
01101101B
00101011B
01100111B
11100111B
00101100B
11101111B
00101111B
=0
=1
=2
=3
=4
=5
=6
=7
=8
=9
One way to do this would be to create an
array of binary numbers that would correspond to each digit.
Begin in this program with the
"dgt()" subroutine on line 21 of this
sketch. The MCP23017 expander
chip has registers. Writing to these
Registers controls the operation of
the of the device.
This subroutine will send a byte
(calling parameter N) to one of the
registers (calling parameter Reg.)
Below is a table of the registers in
the Chip. Register 0x00 and 0x01
determine which pins in port A or B
respectively. Each bit corresponds to
a pin in the port. For this all the port pins must be OUTPUT. To make them such the program
writes 0x00 to each of these registers.
Just as the pinMode()
instruction from the
Arduino this does not
set the values, it only
makes the pins
OUTPUT.
The equivalent of the
digitalWrite()
instruction requires
writing to GPIOA and
GPIOB registers 0x12
and 0x13. Each of the
other registers performs
some other alteration of
the Registers. A
complete listing of the
functions of these
control registers can be found in the data sheet for the chip. Registers 0x0C and 0x0D, the
GPPUA and GPPUB turn on the internal pull up resistors for the pins that have been configured
as inputs.
There is more information on the MCP23017 in the lecture material "Advanced Arduino
Programming 2014 Conference."
More Things About the Counting Program
First the global nmbr[] array. This set of binary numbers will turn on the segments to make the
numbers that corresponding to the position in the array - the 0th element is 0. Element number 6
is 6 and so on.
The setup() routine initializes the I2C
(Wire.begin()), then sets the IODIRA
and IODIRB registers in the
expander chip.
In the loop() function the digits are
calculated and the routine sequences
through the numbers 00 to 99. The
counting variable 'i' is divided by 10.
The ones digit is the modulo of the
counting variable and 10.
This is a simple demonstration of
what can be done with the expander
and couple cheap 7Segment digits.
It's interesting to note that the data
sheet says that the expander can put out 25mA per pin. This is not enough to burn up most LEDs.
This means that series resistors are not essential. However, it is always a good idea to control the
current through the LED and that means good design practice to include them.
Let's look at the schematic for the
expander board. The GP pins are
connected to the control pins through
150 ohm resistors.
This schematic is converted into a
board layout. In this the blue traces
are on the bottom. Red
is on the top.
This converts to a small
circuit board. that
creates the electrical
connections this fits nicely into a small box that can be designed
and printed.
Conclusions
This makes a really nice little display if
the project needs something like this.
As always, if there are any questions, feel
free to contact me.
ak@thecije.com