November
2000 , Issue 124
A
PIC17C44-Based Computer
by
Duane Perkins
According
to the market, single-board computers and PIC microcontrollers
don’t mix. But, it would be nice, wouldn’t
it? If you agree, tune in, because Duane found the
right mixture so we can execute from external RAM.
Single-board
computers that use PIC microcontrollers are not readily
available, partly because all except the top-of-the-line
PIC17Cxx microcontrollers do not allow execution
from external memory. This precludes execution from
RAM, which means that a program cannot be downloaded
and executed. However, the PIC17Cxx microcontrollers
can be configured for a microprocessor or extended microcontroller
mode, much like the Motorola 68HC11, which greatly facilitates
program development. In this article, I’ll explain
how to build a PIC17C44-based computer that can be programmed
to execute from external RAM. I call it the SBC17C44.
Although
the SBC17C44 is designed around the PIC17C44, any of
the PIC17Cxx microcontrollers with 8-KB or smaller
program memory can be used. There are two boards, the
motherboard and daughterboard. The motherboard includes
the external memory, address latch, and device selection
decoder. The daughterboard provides the device interfaces,
including RS-232/422/485 serial I/O level conversion,
an LCD, a keypad, and a piezo buzzer. It also includes
the power supply. The boards are large with wide traces,
making it possible to etch the boards at home using
the simplest possible tools and materials.
