Z80 reset circuit

I need to make up a reset circuit to reset, on power on, a Z80 (old

Does such a modern part exist?

Thanks

Do a search for "microcontroller supervisor". Microchip makes several flavors, one of which will probably handle your needs.

TC1270 ? Includes a manual reset pin, which may be handy here. Digikey says they are in stock, at about $1 each. Another option, that is if Microchip development tools are already set up and ready, is to use a PIC12F629, which includes a BOR and POR (and WDT), 128 bytes of EEPROM, has a comparator, timer, internal precision clock so an external one isn't required, works from 2V-5.5V, is about 10-15 cents cheaper in ones, probably more readily available as it is in wider use I think, and includes the ability to add other special features. Of course, there is the programming problem. So back to the $1 part for one-off simplicity.

Jon

Sorry, it's 30 cents more expensive in ones! I had picked up some on a special deal, which isn't available anymore. So that's old news.

Jon

Please remember that we're still in a recession, Jon.

Bob

And will be still, for some time.

Oh, well.

Jon

P.S. What I'd like to see, micro-wise, is a 6-pin SOT23-6, programmable level BOR, good quality POR, comparator, internal vref, 4 I/O (guaranteed high impedance during reset), small EEPROM, 32kHz internal low power (

I have came across this chip TL7705 a few times and I think this might be what you've been looking. It has active high and active low reset outputs and just required 2 cap and a 10K resistor.

The datasheet is here:

Allen

MAX6326, MAX6327, MAX6328, MAX6346, MAX6347, MAX6348

Industry's Lowest-Power Reset IC Consumes Only 500nA with No External Components Required

also try Maxim:

MAX706 is an old part, still in DIP and they give free samples on 24 hour delivery

-- Bill Naylor

Do it the old way with an RC to the reset pin !

I wondered about that. I'm too lazy to dig in the box to find a Z80 book, so I wondered if there was something more complicated about the reset. There was a special IC for the 8086, but that included the clock, so who knows. I don't even know where I put the Osborne book that introduces the

But yes, a lot of CPUs just required a temporary grounding to reset. They even included a schmidt trigger so bounce wasn't a problem. That then made it easy for the introduction of reset on power on, just a resistor to the +5v line and a capacitor to ground.

You'd even see that, computers would arrive that required one to press the reset button when power was turned on, and people would write about adding a resistor and capacitor to have it reset on startup. I seem to recall doing that with one single board computer, maybe my OSI Superboard, but I can't remember.

The only way this wouldn't work is if the reset pin doesn't have a schmidt trigger, in which case an external one might be needed, or if for some reason the Z80 needed some more complicate reset system (which seems hard to believe).

Michael

Every Z80 I ever had did it with an RC to the reset pin ! It has become more complex since. As you rightly say the X86 also required T to be reset as well.

I don't remember those but I do remember wiring a reset switch to most of the earlier machines including the 8088/8086 and Amstrad 1512/1640 that never had one factory fitted.

I vaguely remember one of the early machines that used to blow up the PSU if you switched it off and on again without waiting for 30 seconds or so.

Agreed !

Tsssk... A reset only needs a cap and a resistor, what do you need chips for??

To make the circuit actually reset?

It is a real pain in the ass to use that RC circuit, when you have multiple chips that need reset. I have had to add a reset generator to equipment that had been used a few years in the field. The RC reset has a ramp. That means that different chips reset at slightly different points. I had some $5,000 edit suite interface systems for 1" Sony broadcast video tape equipment that had become unstable. I looked at the schematic, and saw a RC reset circuit. The sad thing was that the PC board was designed for a 555 reset circuit they had deleted.

It's a Z-80, which is really persnickety when it comes to rise/fall times of its inputs. I remember the heroics we used to have to do just to clock the damn thing!

Cheers! Rich

Come on Rich, we clocked the baby with a debounced push button! There was nothing to it at all.

Simple R-C reset circuits are notoriously unreliable. A good reset should be held *AT* reset during all conditions where the circuit may be active (but perhaps not functional) until power is stable. A simple R-C circuit cannot do this.