I bought tree TI99 4A a year ago and have just gotten them up and running.
The design is a mixture of game cartraige consule and a computer.
There are lot of different cartigage and expation hardware availeble.
Following my prinsiple I wanted to make a MMC disk to this machine.
Searching the huge Internet a found that some one else had done it and put i in Public Domain.
Looking at the design it look like a good but I wanted to redesign is some.
Wanted to fit everything into the original cartiage ans have it reprogramable in system.
All componets are SMB, but can be soldered by hand.
Link to my design
Programing with AVR ISP
I use a USB AVR ISP adapter bought from Aliexpress (USBasp_H6) with PROGISP (Other works).
Status 17 April 2017
Cards has arrived but missing one component.
Programing the card with ISP works
After a soem exprementing a found the documentation had some mis leading iformation.
the correct fuse values are LowValue:E4, HighValue:DF. ExtValue:0, LockValue:FF