I would like help designing a Color Computer shield for the Arduino Mega.
For those who don't know the CoCo is a M6809 based computer from the 1980s.
I imagin a shield with a 40 pin card edge connector (for the CoCo cartridge port) on one end, and header pins for the Arduino on the other end.
I am hoping to design a very passive shield with little or no configurability. But I don't want to limit the kinds of projects avaiable.
Here is the CoCo cartridge port:
A0-A16 - The 6809's address bus.
D0-D7 - The 6809's data bus.
E Clock - Normally a 0.89 MHz clock. The clock generator in the CoCo is software selectable to double this.
Q Clock - Clock which leads E by 90 degrees.
CART* - a signal to the CoCo to immediaetly jump to ROM mapped in CTS* space.
+5 VDC - +5 volts DC, 300mA.
R/W* - Read/write signal from the CoCo.
CTS* - Cartridge (ROM) select. This goes low when the CPU is reading from $C000-$EFFF.
SND - Analog input routed to the TV speaker.
SCS* - Spare cartridge select. This goes low when the CPU is reading or writing to/from $FF40-$FF5F.
SLENB* - When put low by the external device it will have full control of the data bus. This allows overriding of internally (in the CoCo) mapped devices.
What would be a sane mapping to the Arduino's pins?
Some projects I have in mind:
? ROM emulator. Tied to CTS*.
? Data aquisition from the Arduino's analog input. Prolly tied to SCS*.
? Emulate a UART.