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   Snapshot date: 2025-02-12
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Thanks to Takashi TOYOSHIMA for his work on the CP/Mega88 project which can be found here: https://github.com/toyoshim/cp-mega88. Please visit his project page and read through his other work.

The CP/Mega88 project is a i8080 emulator written in AVR assembler with modules in C and assembler to support external static RAM, SD-card and UART, as well as a virtual “monitor” linking it all together.

My rendition focuses only on the AVR side of things, The original project also runs on several other platforms.

Specifications:

i8080 virtual machine running at 2.5 - 3 MHz apparent clock speed.

128 kByte Static RAM (paged memory with 4 32 kByte pages)

SDSC SDHC compatible SD card interface. no hot-swap

TTL serial console 38400 baud, no handshake

3v3 or 5v FTDI or compatible interface

5v power at serial interface or 3v3 at ISP connector

dual boot

CP/M 2.2, 59 kByte TPA

CP/M 3.0, 61 kByte TPA, banked BIOS

4x 8MB disks (A:-D:) shared by both OS-es

Here are the project files: [[https://retrobrewcomputers.org/lib/exe/fetch.php?media=builderpages:rhkoolstar:cp-mega88.zip|cp-mega88.zip]]

The zipfile contains:

all the AVR source code, make files and binaries.

source code for CP/M 2.2 and 3.0 as well as boot sectors

PCB schematics and gerbers for an SMT board.

A 4 disk image file to be copied to the SD card with system files, source files, tools and various applications in several user area's (which are not guaranteed to run: some use Z80 code, some are os-specific)

The SMT board can be built for less than $4.50, although buying single items or TH parts might turn out more expensive. The included PCB uses SMT components. Alternatively you can wire up a piece of perfboard, using TH parts. (use an AS6C1008 for SRAM and a TF card module)

parts list:

|ATmega8820AU                                  |TQFP-32                |
|BSI BS62LV1027SIP-55                          |SOP-32                 |
|74HC374D (x2)                                 |SOP-20 7.2 mm          |
|TXC20.0 Crystal                               |5032 5mm×3.2mm SMD-4PIN|
|1n4148                                        |miniMELF               |
|AMS1117 3.3                                   |SOT-223                |
|4x100n                                        |0805                   |
|2x22p                                         |0805                   |
|10u                                           |0805                   |
|2k7                                           |0805                   |
|3k3                                           |0805                   |
|TransFlash Micro SD Memory Card Slot Connector|Please examine picture |
|ISP & serial connectors                       |                       |

For SRAM any 3.3v 128k x 8 chip can be used (regular pin layout)\\
I chose the BSI chip, because it was cheapest and was 5v tolerant (2.4V ~ 5.5V).

NOTE: running an ATmega88 at 3v3 / 20 MHz is out of specifications (> 13.3 MHz). I have had no issues, but your mileage may vary.

If you have any suggestions, feel free to let me know.

Happy building

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