June
2000, Issue 107
Low-Cost
Software
Bell-202
Modem
by
Stephen Holland
Imagine
this: rapid and reliable data processing via a low-cost
software modem. With the 50-MIPS Scenix SX MCU,
Stephen designed just such a modem, and it offers
the ease of flash memory and in-system debugging
In
every ATM, point-of-sale terminal, and automated gas
pump there’s an embedded modem whose only purpose
is sending and receiving identity-verification data
and purchase information. These are typical of a broad
range of applications in which "data communications"
doesn’t mean transmitting huge pdf files or other
attachments between PCs.
Instead,
embedded modems for information processing need to connect
rapidly and reliably as well as be able to transmit
and receive relatively small amounts of data at a rate
that a person standing in front of an ATM perceives
as fast. For this, low-speed (1200 and 2400 bps) Bell-202-
and 212-compatible modems are adequate.
Because
embedded modems are often used in places that require
small size and low cost, designers must get a least-cost
implementation that is as close to a single-chip solution
as possible. An attractive solution would be one relatively
cheap MCU with all the modem functions inside, eliminating
everything except a serial connection and the digital
access arrangement (DAA) for the telephone line.
But,
this solution wasn’t available until recently.
Embedded modem designers typically had to go with either
a multichip implementation consisting of an inexpensive
8-bit MCU and an external modem chip, or move up the
complexity and cost curve to 16- and 32-bit MCUs and
DSPs that avoid the silicon penalties by handling the
modem functions in software.
There’s
an 8-bit MCU that performs the types of modem functions
needed by the vast majority of embedded applications
in software. Using a Scenix Semiconductor SX series
MCU, I put together a circuit that, in a form factor
smaller than 2² × 3²,
provides all the basic functionality of a Bell-202 modem,
including FSK generation and detection and DTMF generation
and detection.
The
key to the design is that software modules, or "virtual
peripherals," for each function are loaded into
a fast (10-ns access time) on-chip flash program memory
and executed as needed. This eliminates external modem
and memory chips and additional internal silicon development,
and results in an extremely cost-effective implementation.
In fact, the entire bill of materials totals right around
$7.
