Finished the initial design the 64K address space is divided up as so.
0x0000-0x7FFF RAM
0x8000-0x8FFF Peripheral space
0x9000-0xFFFF ROM
I initially had the ROM at the bottom of the address space and the RAM at the to how every thanks to many people on twitter I was informed that the Reset Vector this is a memory address that contains the first address to execute code from should be contained in ROM (this is something I didn't know before this project). The Reset Vector for this MPU is 0xFFFE right at the top of the address space so the RAM and ROM got swapped.
I decided to use a 3 to 8 decoded to select the peripheral instead of basic gates The reason is if i want to add a display buffer or other device i could take a lead form that sacrificing 4K of ROM space.
_A15 is the inverse to A15 generated by a FET and pull-up resistor so it goes low when A15 is high. I also needed it to select between this and the RAM at the bottom of the address space.
![](https://cdn.hackaday.io/images/3847581583443035699.png)
This the full schematic have been over it several times and believe cleaned up any issues.
This is board almost as it has been sent off to the board house I did some more trace optimizing and managed to remove a load of vias.
As you see in the area I have marked in blue.
So now the PCBs are ordered with JLCPCB so just a waiting game.
wish me luck :)
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