built, worked for a little and then...

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giobbi
Posts: 30
Joined: 11 Jul 2021, 22:12

built, worked for a little and then...

Post by giobbi »

hello,

I finally got the PCB and built my Gigatron TTL. I mixed LS and HCT parts (I used the LS I had). The ROM is v. 5A. I checked all the used parts with my TL866 eprom programmer, including the RAM chip. Apparently every used chip I put was fine.

I used a 6.4 MHz crystal (I didn't find the 6.25 MHz here in Brazil).

I was pleased it worked since the beginning (I get the menu) but immediately I got some issues. Well, at least it boots! :-)

First: the Picture option doesn't show a picture at a time: instead, the three images are smaller than the screen, overlapped (you can see the three images on the screen, sometimes more times every images... do you know the pictures of your son/daughter on your fridge? LOL) and endless scrolling from right to left. Maybe it's the correct behavior? I've seen the 8 bit guys one with the pictures shown one at a time (but he had an older rom version...)

Second: no matter the option (pictures, fractal, etc.), after a while the screen becomes black, then come back, then goes black again, until the computer freezes (the blinking sequence stops suddenly).

I was wondering if it could depend on the RAM chip... did anybody experience a similar issue?

cheers,
Giovi
at67
Posts: 391
Joined: 14 May 2018, 08:29

Re: built, worked for a little and then...

Post by at67 »

giobbi wrote: 06 Aug 2021, 00:56 First: the Picture option doesn't show a picture at a time: instead, the three images are smaller than the screen, overlapped (you can see the three images on the screen, sometimes more times every images... do you know the pictures of your son/daughter on your fridge? LOL) and endless scrolling from right to left. Maybe it's the correct behavior? I've seen the 8 bit guys one with the pictures shown one at a time (but he had an older rom version...)
It's supposed to work like that in ROMv5a.
giobbi wrote: 06 Aug 2021, 00:56 Second: no matter the option (pictures, fractal, etc.), after a while the screen becomes black, then come back, then goes black again, until the computer freezes (the blinking sequence stops suddenly).
I was wondering if it could depend on the RAM chip... did anybody experience a similar issue?
Check your supply voltage whilst it is running, make sure you have 150ns or faster ROM and 70ns or faster RAM.

It sounds like you are losing video sync, but are you losing sync because of timing/voltage or is the gigatron rebooting due to poor supply voltage, noise etc. There are many issues that could be causing your symptoms. I would monitor the supply rail with a multi-meter or better yet a CRO and try and catch the gigatron during one of the losing sync cycles.
giobbi
Posts: 30
Joined: 11 Jul 2021, 22:12

Re: built, worked for a little and then...

Post by giobbi »

It's working fine, now, so I have two options:

1) even if I dried it well after I washed with alcohol, there was still some little humidity (really strange, though);
2) I was using my regulated power supply and I set about the current limit to 700mA. It shouldn't make any difference, since it's consuming about 370-380 mA.

However I tried the day after, with the board totally dried, raising the limit to 1.2A, and it worked flawless. I'm running it from my PC USB port and it's working.

both explanations don't satisfy me... but it works now, so .. who cares, LOL

Thanks for the help.
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