A constant pulsing reset is usually a watchdog at play. Hardware watchdogs are usually implemented in systems to reset everything should the system not meet one specific criteria: eg cpu touch one memory address before X amount of time, or pcb voltage lower than X volts, etc.
Watchdogs are also usually found as software routines executed by the cpu also looking for specific conditions. These rarely issue a reset hardware signal, just restar the program. Sent from my iPhone > On 4 Nov 2018, at 14:10, Rob Jarratt via cctalk <cctalk@classiccmp.org> wrote: > > >> -----Original Message----- >> From: Tony Duell [mailto:ard.p850...@gmail.com] >> Sent: 04 November 2018 12:42 >> To: r...@jarratt.me.uk; Jarratt RMA <robert.jarr...@ntlworld.com>; General >> Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> >> Subject: Re: Datasheet for a NEC Chip in DEC Professional 350 >> >> On Sun, Nov 4, 2018 at 12:37 PM Rob Jarratt via cctalk >> <cctalk@classiccmp.org> wrote: >>> >>> I have posted previously about a DEC Pro 350 I am trying to get >>> working again. At the moment it seems to be constantly resetting the CPU. >>> >>> >>> >>> I have traced one possible path for the cause of this back to a NEC >>> chip for which I cannot find a datasheet. It is a 40-pin DIP it is >>> marked "NEC Japan >>> 8239K6 D7201C". All I have been able to find is more modern USB host >>> controllers. >> >> Almost certainly a uPD7201 multi-protocol (asynchronous and synchronous) >> serial chip. I have an NEC data book with it in if all else fails but a >> google >> search for 'uPD7201 datasheet' (no quotes) found sites with the data sheet >> to download as a .pdf file. >> >> Quite why that should reset the machine is beyond me.... > > I have been trying to find what is driving this path in the logic and this > chip was the only one I for which I couldn't identify the pins, but it seems > that from this datasheet > (https://datasheet4u.com/datasheet-pdf-file/1098405/NEC/UPD7201/1) they are > all inputs and not outputs. So I need to look again for an output pin that is > driving this signal. > > Thanks > > Rob >
