Designing Embedded Hardware, 2nd Edition

Chapter 16. 68HC11

You could not step twice into the same river;

for other waters are ever flowing on to you.

—Heraclitus, On the Universe

In this chapter, we'll look at the Freescale Semiconductor (formerly Motorola) 68HC11, a processor architecture that goes back to the very early days of microprocessors. I have a soft spot for this architecture. I first learned to write assembly language on a machine based on a 6802 processor, and I can still remember many of the opcodes by heart and can "read" raw 6800 machine code as though it were source.

The architecture is far from cutting-edge. But it's easy to program, easy to build, and has been stable for a very long time. It's a good platform to start out on, and it's quick and easy to throw together a simple 8-bit computer using these chips. Let's start by taking a quick overview of the processor architecture.

Architecture of the 68HC11

The MC68HC11 is a member of the 8-bit, 6800 microprocessor family. The 68HC11 is a high-density, HCMOS microcontroller unit (MCU) featuring a fully static design. It is essentially a standard 6800 processor (with some enhancements) combined with inbuilt peripherals, such as an enhanced 16-bit timer with four-stage programmable pre-scaler, a serial peripheral interface (SPI), a serial communications interface (SCI), an 8-bit pulse accumulator, real-time interrupts, onboard static RAM, an eight-channel ADC, and onboard EEROM.

The main registers of the MC68HC11 are shown in Figure 16-1. (Note that this ...