RTC help for SVI 738 MSX2 upgrade

By lkpalwa

Expert (114)

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09-04-2018, 22:50

I have one SVi 738 that I upgrade to MSX2, I now want to add the RTC logic Smile
But I get black screen, I using the original SVI clock schematics, I have not added the
battery yet, if I use the roms without RTC support it works fine, so maybe I using incorrect
EXTROM + RS232 (I know the RS232 works as it should, because I use that same rom on other machine, also
the MSX2 main rom is OK because I using version 2.1 and an another strange thing is I setup all same roms
on BlueMSX emulator .. it works fine Smile so it have to be something with the schematic.
Thankful for any help Smile or ideas (any ideas NYYRIKI ?)

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By lkpalwa

Expert (114)

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10-04-2018, 01:13

LOL Smile solved - bad 74LS175 chip, replace the chips and voila it works Tongue
I need to adjust the schematics, 1 CR2032 with a 1N4148 diod is to near 2.2V Sad
So I tested with 2 in serie with Z diod on plus and voila 4.85V and it keep the date/time

Happy now .. another SVI upgraded with RTC Smile2 Smile2 Smile2


By gdx

Enlighted (5592)

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10-04-2018, 01:21

I would like to do it in an MSX1. Do you know if it works without Sub-Rom under DOS?

By lkpalwa

Expert (114)

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10-04-2018, 01:57

I do thinks so Sad, according to the docs the RTC code is EXTROM and uses following
BIOS call rutin REDCLK and WRTCLK (located in 0x01F5H and 0x1F9H)
But I can be wrong .. what kind of MSX1 do you have PHILIPS/SVI/YAMAHA can be upgrade) Smile

By jltursan

Prophet (2619)

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10-04-2018, 09:19

Just for the record, using a schottky diode with a CR2032 gives enough power to keep the RTCs working. The Schottky drains a lot less current (about 0.3V) Wink

By fernando3k

Rookie (24)

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06-08-2018, 09:12

lkpalwa: What is the original scheme on which you based for the RTC circuit?
I have a SVI-738 converted to MSX2 and I would like to add the original RTC circuit to it.