V9990 hw issues

Champion (437)

30-07-2021, 17:20

Quite a while ago I was able to obtain a single V9990 chip and only one of them. As I am not familiar with it and I only have one I decided to build a V9990 module so I can re-use if needed.
The module itself and the card for my DIY MSX to test it with do work and I am able to talk to the V9990 and read/write registers and VRAM but there is no picture most of the time...

It seems that the various sync signals come and go and seem to change in frequency.
Normally when you power on a V9990 you get a screen with a solid color. I mostly cannot get a sync.
Sync can come and go. If a load something in VRAM, every once in a while it is visible for a while but never very long or stable. I have checked both 14.318MHz and 21.x MHz clocks, they are there on the pins.

Any ideas? It could be my luck that I have a flacky/marginal chip.
Does the VRAM have effect on the ability to sync? I've already reflowed both the V9990 and the VRAMs.

By Metalion

Paragon (1529)

31-07-2021, 21:18

A resistor is sometimes needed in the SCART cable, depending on the sensibility of your monitor RGB inputs.

See here : https://www.msx.org/forum/msx-talk/hardware/v9990-tmt-logic-...

By Mumbly

Expert (96)

31-07-2021, 22:16

Vrams are very sensible indeed, if you combine hsync and vsync
To generate the sync signal on a scart, some resistors needs to be
Used as mentionned in Metalion message.
Did you check vsync and hsync signal output on v9990
output pins with a digital oscilloscope ?

By lintweaker

Champion (437)

01-08-2021, 09:22

I test with both a VGA connection with separate H and V-Sync connected and CVBS video out via a CX2075M.
If there is some kind of picture via the CVBS video it is completely out of sync.
Via VGA I sometimes get the correct picture, but only briefly. Most of the times there is no sync (I am using either a GBS converter or direct VGA connection).

I checked all the sync and RBG signals on the V9990 with a scope. Here you can see that for example the HSYNC signal is completely absent (always high) at times. Switching between the 21.5MHz and 14.318MHz clock sometimes helps.
My power supply show the current drawn by the system, you can clearly see a higher current when the V9990 is working for a bit.

By lintweaker

Champion (437)

01-08-2021, 15:13

After watching a Ben Eater YouTube video about a DIY VGA interface I got a clue ....
Managed to get a stable picture out of my V9990 board by connecting CSYNC to pin 13 of the VGA connector instead of HSYNC. Maybe my HSYNC pin is flaky or only working in certain modes.
Thanks for the suggestions

By Mumbly

Expert (96)

01-08-2021, 20:34

No hsync is working on every mode, for sure

By Mumbly

Expert (96)

01-08-2021, 20:38

When I’ll be back to home (currently in vacation) I’ll check
What I did to get a correct sync with my v9990. (vga and scart)

By lintweaker

Champion (437)

02-08-2021, 16:15

Made a bit more progress. The /VRESET and /HRESET pins can be inputs or outputs as part of the color bus.
I did not have either pins connected. After pulling them up to 5V the sync is fully stable. Still have to measure if this had a positive effect on the HSYNC signal.
Normal video out (CVBS) via the CXA2075M is still not working properly but RGB output is solid.