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PostPosted: Sun Mar 19, 2017 7:39 pm 
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Need help about gameboy pin30 /reset signal:

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


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PostPosted: Sun Mar 19, 2017 7:46 pm 
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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.


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PostPosted: Mon Mar 20, 2017 12:37 am 
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If set /reset as input on a cpld cartridge, the cartridge can't boot on GB, but on GBC or GBA is OK.


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PostPosted: Mon Mar 20, 2017 10:59 am 
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The pin has to be allowed to float high...


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PostPosted: Tue Mar 21, 2017 5:15 am 
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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).


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PostPosted: Tue Mar 21, 2017 11:30 am 
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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)


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PostPosted: Wed Mar 22, 2017 6:40 pm 
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/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?

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PostPosted: Wed Mar 22, 2017 7:06 pm 
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on my GB,play a cartridge:
Image
Image
Image


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PostPosted: Wed Mar 22, 2017 7:12 pm 
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It may not be the LR35902 itself, but there's definitely something keeping /RESET from rising until 8.6ms after +5V has stabilized:


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dmg_reset_starts_rising_noticeably_after_5v_is_stable.png
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PostPosted: Mon Apr 03, 2017 5:57 pm 
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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.

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PostPosted: Mon Apr 03, 2017 6:56 pm 
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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.

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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)

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


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PostPosted: Mon Apr 03, 2017 7:20 pm 
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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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