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Need help about gameboy pin30 /reset signal
http://forums.nesdev.com/viewtopic.php?f=23&t=15689
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Author:  byemu [ Sun Mar 19, 2017 7:39 pm ]
Post subject:  Need help about gameboy pin30 /reset signal

Need help about gameboy pin30 /reset signal:

Is it /reset signal is a input signal.
How it works?

Author:  lidnariq [ Sun Mar 19, 2017 7:46 pm ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

I would assume that the /RESET signal is an open-collector bidirectional signal; grounded by the LR35902 until it's ready to start executing code, and probably ground-able by the cart to cause the GBCPU to reboot.

Author:  byemu [ Mon Mar 20, 2017 12:37 am ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

If set /reset as input on a cpld cartridge, the cartridge can't boot on GB, but on GBC or GBA is OK.

Author:  lidnariq [ Mon Mar 20, 2017 10:59 am ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

The pin has to be allowed to float high...

Author:  byemu [ Tue Mar 21, 2017 5:15 am ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

Yes, when I setup a pullup resistance on the /reset signal,it can boot on GB.
BTW, is there a reboot command yet? for example write 00 to FF50 ,then jp $000´╝îsystem will reshow Gamboy LOGO (sorry it's not work,only show a possible).

Author:  lidnariq [ Tue Mar 21, 2017 11:30 am ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

There's no software way back into the Gameboy bootstrap. ($FF50 is write-once). You should be able to just jump back to the DMG's reset location after the bootstrap finishes, though. ($0100)

Author:  nitro2k01 [ Wed Mar 22, 2017 6:40 pm ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

/reset is an input on the CPU. It's weakly pulled up to Vcc with something like a 100k resistor. Probably, your FPGA is sinking the voltage too low. Try adding something like a 10k pull-up on your cart and see if it improves stability.

lidnariq wrote:
grounded by the LR35902 until it's ready to start executing code
Have you confirmed this behavior or is it conjecture?

Author:  byemu [ Wed Mar 22, 2017 7:06 pm ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

on my GB,play a cartridge:
Image
Image
Image

Author:  lidnariq [ Wed Mar 22, 2017 7:12 pm ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

It may not be the LR35902 itself, but there's definitely something keeping /RESET from rising until 8.6ms after +5V has stabilized:

Attachments:
dmg_reset_starts_rising_noticeably_after_5v_is_stable.png
dmg_reset_starts_rising_noticeably_after_5v_is_stable.png [ 3.15 KiB | Viewed 677 times ]

Author:  nitro2k01 [ Mon Apr 03, 2017 5:57 pm ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

lidnariq wrote:
It may not be the LR35902 itself, but there's definitely something keeping /RESET from rising until 8.6ms after +5V has stabilized:

That looks weird. 1) How the power seems to rise linearly rather than converge exponentially. (Capacitor charge curve.) 2) How the /reset signal DOES have a capacitor charge curve type curve.

On a DMG (unless I've missed something!) the only things connected to the reset line are the CPU, a pull-up resistor and the cartridge bus. To get a rise time of 50 ms, there would have to be a ~100 nF capacitor present, which afaik doesn't exist on the boar, but might exist on some cartridges? I'd have to look into this myself. Definitely seems like something holds the /reset line until the power is stable though.

Author:  lidnariq [ Mon Apr 03, 2017 6:56 pm ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

nitro2k01 wrote:
That looks weird. 1) How the power seems to rise linearly rather than converge exponentially. (Capacitor charge curve.)
I have to assume it's an artifact of the flyback converter used inside the DMG. Evidently the flyback counts as a constant-current source charging a capacitor, hence the linear rise.

Quote:
On a DMG (unless I've missed something!) the only things connected to the reset line are the CPU, a pull-up resistor and the cartridge bus. To get a rise time of 50 ms, there would have to be a ~100 nF capacitor present, which afaik doesn't exist on the board
... now that's a very good question. I think your math is a little off (the time constant looks to me like it's somewhere around 15ms, so it's an unexplained 13nF, but 13nF is still too big to be plausibly on-die in late 1980s technology)

Quote:
but might exist on some cartridges?
I did this test without a cart.

Author:  nitro2k01 [ Mon Apr 03, 2017 7:20 pm ]
Post subject:  Re: Need help about gameboy pin30 /reset signal

lidnariq wrote:
I think your math is a little off (the time constant looks to me like it's somewhere around 15ms, so it's an unexplained 13nF, but 13nF is still too big to be plausibly on-die in late 1980s technology)
I would say the time constant is closer to 20 ms (two divisions) but around that ballpark, yeah. By plugging 0.1 and 180000 into this calculator, I get a time constant of 18 ms. 13 nF would give a much shorter TC given the 180k resistor on the board.

http://www.digikey.com/en/resources/con ... e-constant

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