MAD-1 connections on 1A3M-30 PCB - confused

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jaskamakkara
Posts: 5
Joined: Fri May 26, 2017 8:01 am

MAD-1 connections on 1A3M-30 PCB - confused

Post by jaskamakkara » Fri Dec 11, 2020 5:35 am

Hi,

For my own education purposes I have been using my multimeter to map out a bunch of different PCBs. I have a copy of Black Bass III (Jap) which uses a 1A3M-30 PCB. Here is the pinout of the MAD-1 as I can tell with my meter:

Code: Select all

MAD-1:
X---------|1 {/RomCS1}  {/RomCS0} 16|---------X
SRAM /CS1-|2 {/SramCS}    {Addr0} 15|-------A15
X---------|3 {/AuxCS}     {Addr1} 14|-------A20
ROM /OE---|4 {/RomCS}     {Addr2} 13|-------A21
SRAM Vcc--|5 {Vout}       {Addr3} 12|-------Vcc
Vcc-------|6 {Vcc}        {/CART} 11|-----/CART
R1--------|7 {Vbat}        {MODE} 10|-------GND
GND-------|8 {GND}       {/RESET}  9|----/RESET
Notice Addr3 is connected to Vcc, in my experience this is normally connected to A22 since that maps the SRAM correctly to $70-7D,$F0-FF:$0000-7FFF. However, unless I am completely mistaken, if we connect Addr3 to Vcc won't we be also mapping SRAM to $30-3F,$B0-BF:$0000-7FFF (since we don't care about A22 any more)? This area is reserved for all sorts of stuff so why would the cart map the SRAM here?

Is it something to do with the fact that this is a 32Mbit ROM? There is some weirdness with LoRoms of this size since the last bit of ROM cannot be accessed in the lower half of the memory map (because WRAM is mapped in the 7E-7F banks I think) so you have to look for it in the upper mirror (FE-FF).

lidnariq
Posts: 10240
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: MAD-1 connections on 1A3M-30 PCB - confused

Post by lidnariq » Fri Dec 11, 2020 12:15 pm

jaskamakkara wrote:
Fri Dec 11, 2020 5:35 am
This area is reserved for all sorts of stuff so why would the cart map the SRAM here?
Absolutely no idea. There's no advantage to doing so, because nocash's truth table for the MAD-1 says that Addr3 is only for enabling RAM/DSP, and doesn't have anything to do with ROM address ranges.

On the other hand, programs can simply choose to not access those pages in those banks and the nascent bus conflicts could be irrelevant...

jaskamakkara
Posts: 5
Joined: Fri May 26, 2017 8:01 am

Re: MAD-1 connections on 1A3M-30 PCB - confused

Post by jaskamakkara » Fri Dec 11, 2020 12:27 pm

lidnariq wrote:
Fri Dec 11, 2020 12:15 pm
There's no advantage to doing so, because nocash's truth table for the MAD-1 says that Addr3 is only for enabling RAM/DSP, and doesn't have anything to do with ROM address ranges.
My thoughts exactly, it doesn't make much sense to me.

But yeah I guess the solution in these cases is that the ROM simply never looks there so there are no conflicts :shrug:

jaskamakkara
Posts: 5
Joined: Fri May 26, 2017 8:01 am

Re: MAD-1 connections on 1A3M-30 PCB - confused

Post by jaskamakkara » Sat Dec 12, 2020 5:03 am

Thinking about this some more and since /CART only goes LOW in these addresses:

00-3F,80-BF:8000-FFFF, 40-7D,C0-FF:0000-FFFF
(according to nocash)

Then I think that the extra SRAM mapping won't matter since the SRAM won't turn on inside $30-3F,$B0-BF:$0000-7FFF. So maybe they tied that MAD-1 pin to Vcc simply to save having to route another trace on the PCB? I don't know, it's weird. And if this is the reason, why didn't all the carts do this? I have also now checked an SHVC-1A5M-11 cart and it has the same wiring. Something to do with only having one Mask ROM? Eh, as I said, I don't know but if anyone does then please enlighten me!

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