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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
dmg_reset_starts_rising_noticeably_after_5v_is_stable.png [ 3.15 KiB | Viewed 119 times ]
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