Interfacing to large switch banks requires more than stringing a few additional wires
Scott Rosenthal
September, 1995
The tendency for many engineers is to put off designing the user interface until the
last moment figuring: How hard can a few switches be to set up? Unfortunately, with
embedded development routinely making this assumption is an open invitation to disaster.
For instance, consider the following scenario:
A client has a switch panel he wants you to connect to a computer. The panel's maximum
size is 3' wide and 2' high, and is expandable by adding from one to ten columns. Each
column uses a membrane switch pad containing 50 switches and 50 associated LEDs. To make
the pad inexpensively, he's run one wire per switch and one wire per LED to the end of a
flex cable. One common wire handles the switches and another handles the LEDs. The switch
pads and switch panel are already designed and fabricated, and he'd like you to simply
connect the switch panel into a computer. The computer must detect each switch press and
light the corresponding LED. Finally, the system must be rugged, reliable and cheap.
When speaking of electronic hardware, 50 of anything is a lot, but 500 of something (50
switches x 10 columns) is ridiculous! The solution to the previous requirements lies in an
understanding of switches, LEDs, cables and the intended application. The first part of
this column is a refresher on bringing switch signals into a computer. This topic
gradually builds to a system for implementing the above requirements.
Reading the switch
Nothing's more basic in an embedded system than reading a switch. It seems so
simple-just connect the switch to a computer's input pin and then read it. But as with
everything else in the embedded (real) world, there's always some catches. The first one
is that a switch, by itself, won't switch logic levels. It needs a pull-up resistor to
force its output to a High state while its pole is open (see figure). When the switch pole
closes, it connects the input to the computer to ground, or a logic Low state.
Fig-Although the fundamental idea
behind sensing a switch position
involves very basic electronic
theory, servicing a large number of
channels can involve complex design
decisions.
The concept remains simple, but now comes the decision as to what value resistor to
use. The answer depends on several factors including the logic family, the system's noise
environment, the distance from the switch to the computer and the other resistor values
already used in the system.
Different logic families need different currents to correctly set the gate into a High
state. Standard TTL needs 400 µA, so in a typical TTL system, 10ký resistors are about
the largest you can use. With LS-class TTL, its on-chip internal pull-ups remove the
necessity to use external resistors, but being prudent and not wanting to chance a
potential problem, I still add them as insurance. CMOS inputs need essentially no current
to hold them in a High state, but you still have other considerations to worry about.
To begin with, all embedded designs work in electrically noisy environments-remember
all the digital clocks, address and data lines running through the typical system. In
addition, designers often locate switches physically remote from the PC, and a long-wire
run can act as an antenna coupling unwanted signals ranging from hum originating in nearby
fluorescent lights to radio and TV station signals. Hence, a pull-up resistor must be
small enough to keep the High signal at a One, regardless of noise.
In addition, you must keep in mind total system-power dissipation because the pull-up
resistors pass all their current to ground if the switches are closed. In a
battery-powered (or otherwise power-limited) device, switches can contribute significantly
to the overall power budget. Finally, remember that the flex circuits become increasingly
popular because switch connections can have relatively high impedances.
Yet another consideration for the choice of resistors involves what the rest of the
system uses. If you determine that a design needs 22ký resistors, don't go specing this
value if the system uses 20ký resistors elsewhere. For pull-up applications, precision
doesn't usually matter, so be nice to your stock room (and your company's bottom line) and
keep commonalty in the parts lists.
Back to the spec
With this preliminary discussion out of the way, let's return to the original problem
with its 50 to 500 switches. Using the previous discussion as a design guide, these
switches require 500 resistors and 500 input gates to the computer. Assuming 74LS244
variety buffers for inputting all these signals, your system's I/O section will require 63
chips just for reading the switches. Of course this count doesn't include the bus buffers
necessary for the CPU to drive this many peripheral chips. For the 500 resistors, you'll
get a bit luckier and need only 56 10-pin SIP packages.
One potential solution to the problem of burgeoning chip-count is to find a way to
reuse or multiplex the switch signals. But for this solution to work, you must put some
additional requirements on the system. First, the switch panel must respond to only one
switch press at any time. Second, holding down one switch while pressing a second one must
be illegal. Assuming these constraints are amenable, you can significantly reduce the I/O
subsystem's complexity. Referring back to system requirements, each switch pad contains 50
switches, and one switch panel can accommodate as many as ten pads. Multiplexing these
switches reduces the problem from 63 '244 chips down to seven with only six resistor SIPs.
To form the rows for the multiplexing matrix, connect together the like-numbered pins from
each switch pad. The column selection comes from the computer scanning each pad's common
wire.
The scanning process consists of the computer sequentially setting each column to zero
and then reading the row lines to detect the logic Low signal resulting from a closed
switch. This sequence is simple and easy to implement, but given the overall system
requirements outlined at the beginning of this column it won't work. To begin with, the
only reliable way of making all the connections the matrix requires is with a printed
circuit board. When was the last time you saw a circuit board 3' long? Also consider the
added complication (which I glossed over completely) regarding the 50 LEDs/switch panel
and how to drive them.
Micro to the rescue
Of other possible solutions, mine took into account the finished system's target
selling price, customer requirements for reliability and ruggedness as well as the
necessary idiot-proofing. The solution was a small circuit board for each column. Each 2 x
3" board holds a small 8051-style micro, a small Xilinx FPGA and an RS485
transceiver. Each board manages one switch pad, controlling the switch reading and
debouncing, the LED lighting and PC interfacing. The following summarizes some of the
reasoning behind the choices:
Circuit board-Putting local intelligence on each switch pad allows the customer
to easily add additional columns, permits the system to use a simpler (and less costly) PC
and makes for easy field connections. This solution also increases system reliability by
using a simple 7-wire cable for connection between each column and the PC.
Microprocessor-The micro controls the switch scanning, the debouncing, the LED
lighting, the programming of the gate array and communications between the host PC and
other circuit boards.
Xilinx-Although these chips aren't cheap, they pay their way on this project by
implementing the logic that allows the board to maintain its small size. The programming
for the FPGA resides in the micro. On power up, the CPU programs the chip, thereby saving
external ROM.
RS485-This interface allows the system to implement a multidrop system where all
the circuit boards act as listeners. When a board recognizes its address, it responds to
the host PC-a simple interface that works flawlessly.
Simple switches obviously get complicated when strange requirements override their
simple nature. The key is to find solutions that look at all sides of the problem and to
back up the solution with facts, numbers and experience. PE&IN
Adapted from an article that appeared in Personal Engineering & Instrumentation
News.
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