IEEE Spectrum - North American - March 2015 - 24

MAr 2015

NorTH AMerIcAN

eLF In a tIn

a CoSMaC SQUeeZe: this remake
of the 1976 Cosmac elf uses the same
Cdp1802 processor [top] and a strippeddown version of the front panel found on
early micros [second photo from top]. the
processor and panel board are sandwiched
together [third photo]. the computer is
externally powered: I used an arduino as a
5-volt regulated power supply [bottom].

sPecTruM.Ieee.orG

nents, you can install a second RAM chip
or EEPROM. These chips would make boot
loaders and other code permanently available to you. Hart has also included a hefty
capacitor that can maintain the contents of
the RAM chip for several hours.
Squeezing the Elf into a form factor that
can fit inside an Altoids tin means that assembly is a pretty fiddly operation (although most
of the difficulties I had were due to insanely
bad luck rather than Hart's design). And programming software more than a few dozen
bytes long is tedious and error prone.
Fortunately, as of a hardware revision from
last February, the Membership Card now
includes improved support for serial communications, which allows you to load programs
from, and communicate with, a host computer
much as you would with an Arduino.
There are two wrinkles: First, you must
decide in advance if you want to use "TTL"
voltage levels for the serial interface (which
works with many serial-to-USB adapters)
or "RS-232" levels (for interfacing with the
serial ports found on older PCs). The circuitry is slightly different for each. Second,
you'll have to provide your own software to
handle the serial interface (this is where
the ability to install an EEPROM comes in
handy). Fortunately, you can get lots of help
and advice on this-and many other matters related to the Membership Card and
the 1802-on Hart's website as well as on
the pages of Retrotechnology.com.
As I don't have an EEPROM burner, I'm
currently working on an Arduino-based
interface that will connect via the parallel
port provided by the front panel. The goal
is to duplicate the functions of the original
hexadecimal keypad and display while also
allowing longer programs to be uploaded
from a PC, but this will have to be the subject
of a follow-up article. For now I've contented
myself with toggling in some of the sample
programs available for the Elf, including a few
from the original series of articles in Popular
Electronics that introduced the computer
to the world. One of these is ETOPS-256,
a 32-byte-long operating system. It's not
exactly a sophisticated OS, but it is the only
one I can say I have the pleasure of fully
understanding. -Stephen CaSS

from top: stephen cAss (3); rAndi Klett

of external devices, such as serial interfaces
or loudspeakers. Connecting the Q output
to a speaker and turning it on and off at the
right frequencies generates musical tones.
So the Elf was popular among synthesized
music pioneers, as demonstrated by the
Elf's prominence at the First Philadelphia
Computer Music Festival, in 1979.
The 1802 also features four special input
pins. The state of these pins can be read and
branched upon using a single instruction
apiece, greatly simplifying the physical circuitry and software required to accept input
from peripherals. Another handy quirk of the
1802 is that its clock speed can be dynamically adjusted, from about 1.8 megahertz
(in the case of the Membership Card) right
down to zero. Stopping the clock doesn't
cause the processor to reset, and the clock
can be restarted without the 1802 missing a beat. At low speeds the 1802 needs
little voltage and draws only a tiny amount
of power. These features should make the
1802 of interest to anyone who needs a reliable embedded controller that could potentially be run on a small solar cell.
The standard Membership Card Kit, from
Hart, costs US $79-I paid an extra $10 for
a version that includes a cover for the kit's
front panel. Hart also sells a bare-bones
printed circuit board without any components, but buying the components with the
kit means getting an 1802 processor without the hassle of dealing with resellers on
eBay. (Intersil, the current manufacturer
of 1802 processors, sells only a few thousand military-spec versions a year, at $124
apiece, bulk orders only.)
The kit takes the form of two interconnected boards sandwiched together. The
bottom board is actually a complete computer that holds the 1802, system memory, and some supporting circuitry. The
top board is the "front panel" and holds the
toggle switches and LEDs used for input
and output, as well as a power socket and
a 25-pin socket. The latter can be used to
connect the kit to a PC parallel port.
The bottom board can function without
the front panel. It comes equipped with
a 32-kilobyte static RAM chip, but if you
buy some additional third-party compo-

http://www.Retrotechnology.com http://sPecTruM.Ieee.orG

Table of Contents for the Digital Edition of IEEE Spectrum - North American - March 2015