time to create a third color. This allowed me to utilize
three colors (red, green, and orange) with only two actual
LEDs (red and green). Figure 1 illustrates the 8x8 bi-color
LED module connection layout.
Lots of I/O Needed
Being able to control only a single 8x8 LED matrix
was possible with an SX28 chip because one would only
require 20 I/O ports for the anode and cathode LED lines,
as well as related control lines. However, combining four
8x8 LED matrix arrays into a larger 16x16 array would
require 36-40 I/O ports so an SX48 or SX52 chip would
be needed to handle the additional I/O, as well as provide
more program data space and Random Access Memory
(RAM) variable space.
This design uses an SX52 which is now obsolete by
Parallax but can still be found from other on-line sources
(e.g., www.hobbyengineering.com/H2225.html). It may
also be possible to modify the design for use with an
SX48 proto board. The main physical difference between
the SX chips is the available I/O. The SX28 provides 20
I/O ports, the SX48 provides 36 and the SX52 provides
40. All of the Parallax SX chip proto boards are
inexpensively priced. Besides the SX52 proto board, the
only other required SX chip items from Parallax are a
programming tool, being either the USB SX-Key
(debugging capability) or the USB SX Blitz module
(no debugging capability), a 4 MHz resonator, and a
7. 5 VDC one amp power supply.
The key to this project is to arrange the four LED
modules in an expanded matrix to have 16 LED anodes and
16 LED cathodes available to the SX52. I accomplished
this with a point-to-point solder approach so no expensive
PCB is required. This was, however, a bit time-consuming.
Once the 16 LED anodes and 16 LED cathodes are wired
correctly, they can be taken out to the SX52 proto board
to their respective I/O ports of RDE and RBC. A DS1302
Real Time Clock (RTC) chip is used with a 32.768 kHz
crystal also available from Parallax. The RTC and crystal
use three data lines connecting to the SX52 RA I/O ports.
Two other RA I/O ports are used for the LED color control
hardware four (74HC573 ICs) to decide whether to select
red, green, or yellow LEDs via software selection.
Finally, a 74HC165 chip is used with the final RA
I/O ports for up to eight buttons for expansion control.
However, only three buttons are used in this design (e.g.,
up, down, and next) to set the time and date, so it is
possible to redesign this section and wire in three buttons
directly into the 3 RA I/O ports with some software
modifications and not even use the 74HC165 at all.
Besides the SX52 proto board, programmer, four LED
matrix modules, and RTC, the design also requires color and
button control hardware as stated earlier. The color control
■ FIGURE 1
is comprised of four 74HC573 octal D-type transparent
latch ICs. Software control for either red, green, or both
enables the appropriate set of 16 I/O lines going to either
the red or green ULN2803 Darlington array ICs for current
amplification to the 16 LED matrix module cathodes.
Besides the four ULN2803 Darlington array ICs, the
16 LED matrix module anodes require additional current
amplification accomplished by 16 very small surface
mount FDN304PZ P-channel FETs. It would also be
possible to redesign the circuit and use N-channel FETs in
place of the ULN2803 ICs for better current control across
the LEDs. However, for this design it is adequate. I had to
January 2009 41