Memotech MTX Reset Circuit

Introduction

When a CPU is powered up, it is likely that its internal registers will be initialised to random values generated whilst the power supply stabilises at the required voltage. To ensure that the CPU starts up in a known state, a dedicated pin on the CPU is normally used to signal the CPU to perform a reset - for the Z80, this is the active low RESET pin.

To ensure that the reset function has sufficient time to perform the required operations, the reset signal must be held in the active state for a minimum period, defined by the CPU manufacturer in terms of a number of clock cycles. For the Z80 CPU, the Zilog databook advises that "RESET must be active for a minimum of three full clock cycles before the reset operation is complete".

MTX Reset Circuit

To satisfy the Z80's start-up criteria, the MTX included the reset circuit shown :

The MTX reset circuit is used to reset the system following power on or in response to the user simultaneously pressing the two unmarked keys art either side of the keyboard <space> bar, connected to connector J1 pins 19 & 20 on the MTX computer board.

The reset of the circuit is essentially a simple R-C delay which holds RESET low long enough to meet the Z80 specification.

The schematic was drawn using the free edition of TINA Version 9, "a SPICE-Based Analog Simulation Program", available from Texas Instruments. To simulate the reset circuit, a voltage generator and measurement points were added as shown :

Simulating the circuit, with an artificially long time for the 5v supply to reach working voltage, demonstrates how RESET is held low until the supply is stable and the Z80 criteria are met : 

With the system running with stable 5v supply, simulation of the system after the reset keys have been released produces the result shown :

In this case, the RESET is held low for an additional 85ms, well in excess of the 3 clock cycle minimum specified by Zilog.