Gameboy Development Forum
Discussion about software development for the old-school Gameboys, ranging from the "Gray brick" to Gameboy Color
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#1 2021-01-26 12:48:04
- kalt
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- Registered: 2021-01-26
- Posts: 7
GBC Timing Diagram for Cartridge Slot
I'm looking at bitbanging the GBC cartridge slot with a 40MHZ PIC18. Is there a timing diagram that shows over what interval the LR35902 samples the line during a read and during what interval the data is considered valid on the data bus during a write? I assume there is a separate timing for when the CPU is in 8MHz vs 4MHz mode. Even if there is not a full diagram, any insight on bus timings would be helpful.
Last edited by kalt (2021-01-26 22:56:13)
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#2 2021-01-28 20:34:16
- Tauwasser
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- Registered: 2010-10-23
- Posts: 160
Re: GBC Timing Diagram for Cartridge Slot
Check out gekkio's Gameboy Complete Technical Reference, which includes timing diagrams in one of the appendices.
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#3 2021-01-29 10:32:59
- kalt
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- Registered: 2021-01-26
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Re: GBC Timing Diagram for Cartridge Slot
Hmm, Im looking through the document and im not seeing what i am looking for. I see a timing diagram of some sort that shows the cpu fetch cycle. but it gives no information about the where the sampling occurs. i.e. is it on a rising edge? is it on a falling edge? which edge? how long after the transition is the data still asserted on the bus, etc.
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#4 2021-01-29 19:14:06
- Tauwasser
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- Registered: 2010-10-23
- Posts: 160
Re: GBC Timing Diagram for Cartridge Slot
Are you sure you looked at the figures in appendix C? The yellow line is the edge data is sampled on, I believe. IIRC I read in Discord that this differs between regular and double speed mode and these timings would show regular mode.
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#5 2021-01-29 21:48:27
- kalt
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- Registered: 2021-01-26
- Posts: 7
Re: GBC Timing Diagram for Cartridge Slot
ah okay, i didnt know that was what the yellow line symbolized. thanks. When I finally get my breakout board shipped I will check the lines on the oscilloscope to verify.
Last edited by kalt (2021-01-29 21:48:56)
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#6 2021-01-30 16:42:19
- gekkio
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- Registered: 2016-05-29
- Posts: 16
Re: GBC Timing Diagram for Cartridge Slot
IIRC in addition to faster clock, the only difference between double-speed and normal mode is that the address pins are asserted at the same time with A15/CS. I can't remember 100% surely which edge it was, but I'm guessing A15/CS is asserted one edge earlier.
Take this with a grain of salt, because I've only checked CGB timings once...
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#7 2021-02-01 08:33:03
- kalt
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- Registered: 2021-01-26
- Posts: 7
Re: GBC Timing Diagram for Cartridge Slot
thanks, my pin breakout board should be coming in a week or so. It looks like I should be able to bit bang it at 40MHZ provided I choose a ram chip with parallel access that can operate as fast as the controller. I will verify the timings once it comes. In the meantime, I think i have enough to move forward with the board design for the cart.
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