Microprocessor RS-232 Reset

(C) 2002, G. Forrest Cook

Microprocessor RS-232 Break Reset Circuit

Introduction

This circuit allows a remote microprocessor to be reset by a controlling host by sending a break signal over an RS-232 or RS-422 serial line. If the remote machine resets into a simple program loader program, it is possible for the host to halt and restart or halt and reload/restart the remote machine's program. This is an ideal way to develop software on older EPROM-based systems. Modern EEPROM/NVRAM systems use JTAG interfaces for similar results.

The remote processor runs its target code out of RAM, allowing the code to be updated easily. The circuit is usually used for developing code on a target processor, but it can also be used for permanent applications where the target program lives on a host system's disk. This system was once used to load code into a microprocessor-based satellite receiving system in Hawaii from a host system in Colorado using an Internet-based remote serial communications program.

A program loader for the Z-80 CPU is available below. The circuit has also been used with the Motorola 68HC11 EVB and the BUFFALO monitor program. See my Linux Cross Assemblers page for more info.

Theory

The circuit detects long duration zero-level signals (breaks) on a NRZ (non return to zero) serial data line. Normal serial characters spend a short time in the zero state, and do not cause a reset. Break signals are a exception to this, they hold the line low for an extended period of time.

The 1N4148 diode charges the 2.2uF capacitor from the normally-high logic level on the input. Low input signals slowly pull the charge on the 2.2uF capacitor down through the 10K resistor. High input signals quickly recharge the capacitor through the 1N4148 diode. A break signal lasts long enough to discharge the 2.2uF capacitor to the point where the following gate changes state.

The 1N4148 on the right side of the schematic prevents the outputs from the two parallel 74HC14 gates from being shorted to ground by the manual reset switch which is normally wired on the target CPU.

This circuit could be built with just 2 schmidt trigger non-inverting buffers, the 74HC14 was chosen because it is a common part. The parallel inverters are also optional, single inverters will also work.

Z-80 Program Loader

The sbld.mac code is a Z-80 assembly language program that functions as a bootstrap loader for use with this Microprocessor RS-232 Reset circuit. The code runs on an ancient SD systems SBC200 Z-80 circuit board and can be made to work with other Z-80 boards by changing the initialization and serial port functions.

Back to FC's Computer Boot Management Circuits page.