Issue
158 September 2003
The
XY-Plotter
Drive
High-Resolution LCDs For Less
Mad
Dash for Flash Cash Contest Winner
MICRO
OF CHOICE
I
chose the PIC18F252 microcontroller on the basis of
certain project-specific criteria. First, I needed speed.
The more instructions in these bloody 780-ns nibbles
the better. I also wanted a significant amount of RAM.
I didn’t store the full bitmap but chose instead to
store minimum, maximum, and sample values for each column
already requiring 768 bytes. In addition, I needed a
precision A/D converter and a large program memory for
amassing the huge tables used in the design (including
character bitmaps). Lastly, flash memory was necessary
for configuring the display for each application.
One
or two years ago, these requirements probably would
have been impossible to fulfill, but, thanks to suppliers
like Microchip, they are now easily satisfied, with
the PIC18Fxx2 product line in particular. The PIC18F252,
for instance, has 1.5 KB of RAM and plenty of flash
memory (32 KB).
GRASPING
THE SCHEMATICS
Figures
4 and 5 are schematics of the XY-Plotter. Each analog
input (X, Y, AUX1, and AUX2) is conditioned thanks to
half of an MCP6022 dual rail-to-rail op-amp. Two 20-turn
trimmers per input give you the ability to easily adjust
the full-scale deviation as well as the DC offset for
each channel. One of the channels, AUX2, even includes
two inputs summed by the analog amplifier.
|
(Click
here to enlarge)
|
Figure
4—The XY-Plotter’s power supply isn’t included in
this schematic. An MCP6022 analog amplifier, with
scale and offset controls, scales each analog input.
Some of the microcontroller’s I/O lines are multiplexed
to limit the I/O count requirement. |
The
values of the resistors used for each amplifier stage
can be adjusted for each specific application to accommodate
different input ranges and adjustment precision. It
is not obvious how to design an amplifier stage with
positive and negative offset adjustment without a negative
power supply. Here’s my trick: A fixed positive voltage,
which is derived from a 0.6-V reference, is first subtracted
from the input signal, and then a variable positive
voltage is added to it, providing an offset that’s either
positive or negative. I used Excel to calculate the
resistors.
The
PIC is clocked by a 10-MHz crystal up-converted to 40
MHz thanks to the on-board PLL. The LCD is directly
connected to the PIC I/O lines, whereas the auxiliary
digital inputs, which are used to dynamically select
the text for the screen, are either direct inputs of
the PIC or multiplexed with LCD data lines (thanks to
a firmware reconfiguration on the fly).
Lastly,
the ubiquitous MAX232 does what it’s intended to do.
It should be noted that I included an in-circuit programming
header just in case; however, I haven’t had to use it
thanks to Microchip’s boot loader firmware. All of the
programming was accomplished though the serial port.
POWER
SUPPLIES
The
power supply is a significant part of the design (see
Figure 5). First, I needed a clean 5 V. I was already
using all of the PIC’s analog inputs, so I couldn’t
configure its ADC in external-reference mode. I still
needed a stable reference for the analog-to-digital
conversions. After experiencing a few headaches, I decided
to use the PIC in its 0- to 5-V reference mode and to
provide a well-stabilized 5 V. I implemented a high-precision
MCP1541 voltage reference and built a discrete power
supply around a low-drift LMC6462 op-amp. The second
part of the op-amp is used to get the 0.6-V reference
drawn on by the offset circuitry.
|
(Click
here to enlarge)
|
Figure
5—The power supply includes four independent subsystems,
one of which is the main 5-V regulator, which I
built using a high-precision Microchip MCP1541 reference.
I used a 5- to –24-V converter for the LCD. A homemade
converter supplies the backlight voltage (100-V
AC). Lastly, note that a 0.6-V reference is provided
for offset control. |
The
LCD was hard to deal with because it needed both a –24-VDC
input (for the display itself) and a 100-VAC power for
the EL backlight. To limit the number of power inputs,
I went with a small 5- to ±12-VDC converter to generate
the –24 V switched by two transistors under PIC control.
I couldn’t find a ready-made DC/AC converter for the
backlight in time, but it wasn’t an issue. I built a
pretty one with a small 220/12-V transformer driven
by a NE555 timer. Done.
