Minimum circuit to reset a microcomputer when a peripheral is turned off - Page 1
Forum section: electronics / beginners

author: RPX
author_rank: Newbie
timestamp: November 19, 2021, 06:04:10 am
content: Hello,I've got a bunch of design files for some old so called "interfaces" (peripherals) for 80's 8-bit computers and before ordering the pcbs I would like to make some slight modifications.Each of these interfaces has a ROM/EEPROM memory that under certain conditions pages over the internal ROM of the microcomputer; and also, has a push switch which allows to reset the microcomputer connected to the ~RESET signal of the microcomputer bus slot which at the same time is directly connected to the homonymous signal of the microprocessor.I'm thinking to add to each pcb an ON/OFF switch to its power supply (+5V) in order to minimize having to continuously and physically connects and disconnects the peripheral from the microcomputer avoiding stress to the long-suffering motherboard bus slot, but, to avoid problems derived from unforeseen states in the ROM paging, I would like to add a small circuit with the minimum possible components, that reset the microcomputer/microprocessor just at the moment of setting the switch in the OFF position, that is to say, to send a ~RESET TTL pulse when turning off the peripheral.I guess this is a common occurrence in many electronic circuits but, as I'm not in the mood to reinvent the wheel, I just would like to know what the most current industry standards are to solve this little problem. I'd appreciate any suggestions about this matter.

author: ledtester
author_rank: Super Contributor
timestamp: November 19, 2021, 11:01:31 am
content: It seems to me that the absolutely safest approach would be to:- first put the processor in reset- de-power the peripheral pcb- then release the processor from resetThis could be accomplished with two physical switches -- a push button to put the processor in reset for a short time, say 3 seconds, and a slide/toggle switch to control power to the pcb. Correct sequencing of the switches would be the operator's responsibility.The push button would be the input for a pulse generator (aka "monostable multivibrator"). Here's my go-to reference for such circuits written by Ray Marston in 1988:http://www.n5dux.com/ham/files/pdf/Working%20With%20Monostable%20Multivibrators.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: November 19, 2021, 12:54:07 pm
content: Not a good idea. When not powered the peripheral board would load the system bus, probably causing malfunctions and possibly damaging the CPU.To make it work, when the board is powered down, you've got to effectively disconnect the data bus, and ensure that all address lines and control signals used by the board are Hi-Z. when not powered.

author: RPX
author_rank: Newbie
timestamp: November 19, 2021, 03:55:54 pm
content: I appreciate your suggestions.Quote from: ledtester on November 19, 2021, 11:01:31 amI have tried this setting and it works but my ultimate goal is to eliminate the push switch and have the slide/toggle switch perform the two operations, RESET and disconnect the power without any intervention by the operator. I have to think about it more, I will come back to tell what I was able to do.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 19, 2021, 05:22:11 pm
content: I want to give some more weight to Ian.M's remark.You should not input signals to unpowered IC's.IC's generally have ESD protection diodes that shunt the inputs to their power supply rails. This means that the inputs will serve as a powersupply and the "un powered" IC's will draw excessive power from those inputs in an effort to power themselves (and other IC's on the same power rails) and this excessive power draw will load the outputs beyond their specification.I assume you have some kind of bus setup. The proper way to do this is not by removing the power supply of a part of the circuit, but to manage the bus, for example with chip enable signals.