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Par PingPong

Prophet (4093)

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01-09-2010, 19:38

I've read about an X7 pin on S3527 MSX Engine doc.
Docs says that this pin have some connection with VDP wait function.
It's unclear to me. Some one can explain the function of the pin?

Thx, in advance

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Par RetroTechie

Paragon (1563)

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02-09-2010, 01:39

Have a look at the S1985 datasheet, perhaps you're confusing with that one?

Par Bogogil

Master (135)

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02-09-2010, 07:37

As I understood the wait pin of the VDP, it delays the access to VDP/VRAM when the VDP is busy.

Par RetroTechie

Paragon (1563)

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02-09-2010, 08:16

Only V9958 (MSX2+ / Turbo-R) has a /WAIT output pin, V9938 (MSX2) & MSX1 videochips do not.

So this is about the engine sending a WAIT signal to the Z80. I read the S1985 datasheet as 'insert an extra wait cycle on VDP access' (apart from the WAIT cycle that's inserted for every /M1 - Z80 instruction fetch). But what for? Question (only some random, non-founded guesses here)

Par PingPong

Prophet (4093)

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02-09-2010, 09:59

Have a look at the S1985 datasheet, perhaps you're confusing with that one?
the s3527 datasheet is japanese..... but i se the same pin on both s3527 & s1985 so i think the use is the same.

would be nice to know the usage of this pin and if it's connected to something on a real hw.

Par Eugeny_Brychkov

Paragon (1232)

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02-09-2010, 10:38

S3527/S1985 /WAIT pin (output) is connected to the Z80 CPU /WAIT pin (input). As per MSX specification, chip automatically activates /WAIT signal for single clock cycle for every /M1 cycle.

If line X7 is at low (indirectly to reset signal with some delay) during reset, chip will automatically insert /WAIT signal for each access to ports 98h-9Bh allowing VDP to have some more time to read information from its RAM or do another required task reliably.

Par PingPong

Prophet (4093)

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02-09-2010, 11:10

S3527/S1985 /WAIT pin (output) is connected to the Z80 CPU /WAIT pin (input). As per MSX specification, chip automatically activates /WAIT signal for single clock cycle for every /M1 cycle.

If line X7 is at low (indirectly to reset signal with some delay) during reset, chip will automatically insert /WAIT signal for each access to ports 98h-9Bh allowing VDP to have some more time to read information from its RAM or do another required task reliably.
so the next question is:
how it's easy to know, what computer have x7 low during reset? for example NMS8245?

Par Eugeny_Brychkov

Paragon (1232)

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02-09-2010, 11:28

I was wrong. What I said is valid for S1985 only. Please excuse me for misinformation.

In S3527 /X7 level during reset is used to lock up /Y10 scan line mode or serial key mode (note: MSX does not support serial key mode - thus X7 has no sense for you).

In NMS8245 /X7 is in high during reset, thus /Y10 is just another keyboard scan line, connected to GRAPH key through diode.

Par PingPong

Prophet (4093)

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02-09-2010, 11:58

I was wrong. What I said is valid for S1985 only. Please excuse me for misinformation.

In S3527 /X7 level during reset is used to lock up /Y10 scan line mode or serial key mode (note: MSX does not support serial key mode - thus X7 has no sense for you).

In NMS8245 /X7 is in high during reset, thus /Y10 is just another keyboard scan line, connected to GRAPH key through diode.
Me too. the same name X7 fooled me.
So, most probably on the NMS there is no additional W.S. when accessing VDP.
there are some misteries, IMHO about the MSX-Engine chips.
For example i never explained me why the MSX TurboR engine feed a Wait signal to R800 when talking with VDP when the VDP itself is able to signal, more efficiently the same thing. And when turboR works in z80 mode there is the same delay from S1990 or it's from VDP like on msx2+?

Par Eugeny_Brychkov

Paragon (1232)

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02-09-2010, 12:12

So, most probably on the NMS there is no additional W.S. when accessing VDP.
Or additional wait is always inserted.