Designing Embedded Hardware, 2nd Edition

A Simple 68HC11-Based Computer

The computer will have 32K of static RAM, 16K of EPROM, a serial interface (internal to the 68HC11), and a latch controlling a bank of LEDs. While EPROM is old technology, I have chosen it for this system for two reasons. The first is that it is still common for 68HC11 machines to use EPROM, often for historical and legacy reasons. The second reason is that it allows me to show you how to use an EPROM in a design. Following the theme of "showing you how it's done," we'll also do the glue logic for the computer using discrete gates rather than a PAL.

The 68HC11 was designed to be used in a wide range of small applications, many relating to the monitoring or control of external devices. As such, it can run in several modes: single-chip mode, expanded multiplexed mode, bootstrap mode, and test mode. This last mode is used by Freescale during manufacturing and is not intended for user applications. In single-chip mode, the processor relies entirely on its internal features (small RAM, small ROM, I/O, and timers) and has no external address or data bus. The majority of pins (known as ports A, B, C, and D in this particular processor) are therefore dedicated to digital I/O functions. In expanded multiplexed mode, the processor behaves like an ordinary, 8-bit processor, with ports B and C assuming the roles of the address and data buses. In bootstrap mode, the processor loads its vectors from the internal 192-byte ROM and initializes the internal serial ...