Chinese basic educational computer with EX-RAM

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krzysiobal
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Chinese basic educational computer with EX-RAM

Post by krzysiobal »

Chinese basic educational computer with EX-RAM

Cartridge can operate in one of two modes Originally it would switch on each reset, but for unknown reasons, diode and capacitor, needed for M2 filtering were not installed and instead, additional wires
U9.3 - U9.8
U1.2 - U8.13
U1.15 - U8.12
U9.12 - U4.27
U9.11 - U5.22
were added to make this bizarre circuit that switches between modes every time any write to $8000-$ffff occurs
Image


Mode 0 (PRG_A15=0, so PRG_nA15=1)

Code: Select all

PRG: |$8000-$ffff |
     |     0      |
(first 32 kB of the PRG-ROM chip is used)
     

CHR: | $0000 | $1000 | 
     |  A+B  |  A+B  | 
each 4kB bank points to the same region of CHR memory
* first halves (A) of each tile are in CHR-ROM at $00000-$007FF
* second halves (B) of each tile are in CHR-ROM at $10000-$107FF
     
   PPU ADDRESS SPACE      |       CHR ROM layout
$0000 -> tile $00 lo      |    $00000 -> tile $00 lo
$0008 -> tile $00 hi      |    $00008 -> tile $01 lo
$0010 -> tile $01 lo      |           ..
$0018 -> tile $01 hi      |    $007f8 -> tile $ff lo
      ..                  |    $00800 \ 
$0FF0 -> tile $ff lo      |            > not accessible in this mode
$0FF8 -> tile $ff hi      |    $0ffff /
---------------------     |    $10000 -> tile $00 hi
$1000 -> tile $00 lo      |    $10008 -> tile $01 hi
$1008 -> tile $00 hi      |           ..
$1010 -> tile $01 lo      |    $107f8 -> tile $ff hi
$1018 -> tile $01 hi      |
      ..                  |
$1FF0 -> tile $ff lo      |
$1FF8 -> tile $ff hi      |

     
The second (unpopulated) CHR-ROM chip (U7) was meant to be enabled when PPU_A12=0 but that would make a conflict with firstgCHR-ROM chip, because it is always enabled at $0000-$1fff
-----------

Mode 1 (PRG_A15=1, so PRG_nA15=0)

Code: Select all

PRG: |$8000-$ffff |
     |     1      |
(second 32 kB of the PRG-ROM chip is used)


* $2400-$2800 [..CCCCCC] -> each writes in that region goes to EX-RAM 
                 ||||||
                 ++++++- 2k CHR-bank used for the tile (both bitplanes of each sprite are the same)
                
* then when reading from any of the nametables ($2000, $2400, $2800, $2c00), the same value from EX-RAM is used do determine which CHR-bank should be used for tile

PAL equations:

Code: Select all

CHR_A16 =  PPU_A3 when PRG_nA15 = '1' else 'Z' --equivalent to ('Z' & !PRG_nA15) | (PRG_nA15 & PPU_A3);

CHR2_nCE = PPU_A12 when PRG_nA15 = '1' else 'Z' --equivalent to ('Z' & !PRG_nA15) | (PRG_nA15 & PPU_A12);

CHR1_nCE = (PPU_A13);

nGNDBUF = 0

EX_nWE = (PPU_nWE);

EX_nCS = (!PPU_nWE & !PPU_A10) | (!PPU_A13);

PPU_nWE PPU_A13 PPU_A10 | EX_nCS
   0       *       0        1
   0       0       1        1
   0       1       1        0
   1       0       0        1
   1       1       0        0
   1       0       1        1
   1       1       1        0

EX_CLK = (PPU_nWE & PPU_A13 & !PPU_nRD) | (PPU_A10 & PPU_A13 & !PPU_nRD);

EX_nOC = (PRG_nA15);

There is also place for second 74*245 that would drive PPU data bus with zeros, but the PAL lice enabling it is always at zero, so no idea how it oculd be used.
Attachments
top.jpg
sch.png
shell-top.jpg
bottom.jpg
top-components.jpg
Last edited by krzysiobal on Tue May 18, 2021 10:17 am, edited 2 times in total.
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krzysiobal
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Re: Chinese basic educational computer with EX-RAM

Post by krzysiobal »

dumps
Attachments
20210518_135735.jpg
prg in mode1.nes
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prg in mode0.nes
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chr.bin
(128 KiB) Downloaded 60 times
NewRisingSun
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Joined: Thu May 19, 2005 11:30 am

Re: Chinese basic educational computer with EX-RAM

Post by NewRisingSun »

Thank you. I can emulate everything well...
Hongda8in1.png
Hongda8in1.png (2.24 KiB) Viewed 2921 times
... except the "key exercise" screen, which looks strange.
KeyExercise.png
KeyExercise.png (1.12 KiB) Viewed 2921 times
Mind posting a real-hardware screenshot of it so I can know what it is supposed to look like?
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krzysiobal
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Re: Chinese basic educational computer with EX-RAM

Post by krzysiobal »

Does it need subor keyboard to reach this screen?
NewRisingSun
Posts: 1510
Joined: Thu May 19, 2005 11:30 am

Re: Chinese basic educational computer with EX-RAM

Post by NewRisingSun »

No, the cartridge requires Nintendo's original Family BASIC keyboard.
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