Issue
144 July 2002
Driving
the NKK Smartswitch
Part
1: Configuration and Software
by
Aubrey Kagan
Sure,
the NKK Smartswitch looks like a cool device, but
how can you actually use it? Aubrey found the answer
with a little help from a Cypress Microsystems PSoC
microcomputer. In this article, Aubrey describes the
interface to drive the Smartswitch.
Start
•
Hareware Interface • Module
Implementation •
Sources
& PDF
The
NKK Smartswitch is a single-pole, normally open switch
with an LCD screen that serves as its activator. I have
often looked at the advertisement for the Smartswitch
and thought about how cool it would be to use it. But
where and how can the Smartswitch be used? In this series
of articles, I’ll explain my answer to those questions.
The
switch head of the Smartswitch consists of a 24 × 36
pixel LCD and some rudimentary driver electronics that
drive one row at a time. Control signals are required
to shift out the pixel state for each row, latch the
pixel states, and synchronize the first line. In addition,
the display has a backlight that can be configured as
red, green, or yellow. I have tried to make the display
interface pixel addressable to allow for the mixture
of graphics and text to be placed anywhere on the screen.
Obviously,
a microcomputer is needed to drive the display. I wrote
the software so that the user interface is through a
series of RAM locations. This allows display access
to an external hardware interface such as a serial or
parallel driver (based on the microcomputer’s capabilities)
in a stand-alone application or a concurrent task resident
on the microcomputer.
At
this time, I am unaware of any published projects that
use a Cypress PSoC microcomputer. However, I’m sure
this will change in the near future as more users begin
to appreciate the advantages of the PSoC. The versatility
of its I/O configuration, the low cost of its development
tools, and the price and performance of its peripherals
make it an extremely attractive option. In addition,
the PSoC is available in DIP packages, which is great
for low-volume applications and debugging. And that’s
a plus for aging baby boomers who have difficulty seeing
the pins on SMD devices, let alone connecting an oscilloscope
probe to one.
The
LCD
The
LCD is controlled row by row with a 40-bit shift register.
The first four bits shifted out are invisible, but they
must be included in the shifting process. Each pixel
has been allocated a number so that the software implementation
can address a specific location. You can see this allocation
in Table 1. The firmware display driver creates a memory
map where each pixel maps to a unique bit in RAM. As
you’ll see, the map is continuously output to the display
by the drivers.
| Table
1—The numbering of the pixels is arbitrary.
Pixels 0, 40, 80, etc. are the first bits shifted
out for each row. |
In
order to economize on RAM, the memory map is implemented
as a 24 × 5 unsigned character array (cMatrix [24][5]
in the software). Obviously, the first index is the
row number and the second refers to the column number.
Each bit in a byte is allocated to a pixel as shown
in Table 2.
| Table
2—In RAM to pixel mapping, for example, the
most significant bit of cMatrix[1][0] will map to
pixel 79 and the least significant bit of cMatrix[0][2]
will map to pixel 16. |
The
design of the Smartswitch allows for multiple switches
by linking the 40-bit shift registers. Because of the
processor’s RAM limitations, I have left room for only
one switch and one controller. I believe the road map
for the PSoC includes devices with up to 1 KB of RAM,
so it may be possible someday to expand the application.
At any rate, the PSoC’s low cost may allow for a multiprocessor
solution because the switch itself is around $50.
Currently,
PSoC Designer does nothing to enhance this. It allows
for one configuration and then it’s up to you to create
alternatives to overlay these functions. Cypress has
decided that this feature is a competitive advantage,
so they have a new version of their PSoC Designer in
the wings that will simplify this process.
Cypress
produces three kinds of documentation. The first is
the standard datasheet, which at first glance is somewhat
bewildering because of the variety of configuration
options. When you select a module in the user interface,
a datasheet for the module appears on the screen. In
addition to samples of the code, the datasheet reading
includes all the relevant registers and I/O points.
As your product is configured, the PSoC Designer creates
a third datasheet specifically for that particular arrangement
so you have complete documentation of your product.
All
of this is available in an 8- to 64-pin package for
about $5. A full development environment that includes
C costs less than $300. And you’d think they were paying
me to say this, but unfortunately they’re not.
