Implementing this into a CPLD
Based on the features, at the very minimum it needs
- 4 x 8 bit registers for the storage of the counters
- 8 x 3 bit registers for storing the last three states of the DIR & CLK inputs at each clock pulse
- 8 x 1 registers for storing the previous state of DIR & CLK
So it needs at least a 64 register device, and that is without any
extra logic.
Anyway to cut a long story short... after including
all the logic, the result is upto 72 gates plus a LOT of product terms. So alas it will not conveniently fit into a XC9572 CPLD, so next up is the
XC95108 which fits but is, of course, a more expensive part.
This results in bigger carrier board, seeing
as I only had PLCC parts to hand. But does make routing easier with all
those extra pins. It makes soldering easier too with a through-hole socket.
VHDL source
Just the VHDL source code for the LETA replacement
Click for the source
Notes about the implementation
The CPLD was fitted so unused pins were pulled to ground, macrocells were in low-power mode and programmable slew rate was slow.
It was built using Xilinx webpack ise 9.1 and no Xilinx specific features were used, so it should be portable to other architectures.
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Carrier PCB
Having programable pins on the CPLD allowed me to arrange the routing
so it could be fitted easily onto a single sided PCB, with just two jumper
wires.
The ZIP file contains the Eagle 4.16r1 format schematic
and board.
- leta_carrier_PCB_e4.sch - schematic
- leta_carrier_PCB_e4.brd - board layout
- leta_carrier_PCB_e4.bmp
600dpi printer template mask for making the PCB
See MG Chemicals Prototyping Center
Carrier
PCB - board & schematic 
Click to
download
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Click for the BIG picture
Testing !
For testing I did a quick check by plugging it into the test board on
page 1. Then into a couple of different PCB.s.
Tested with
success
- Rampart - 3 player trackball
- Rampart - 2 player joystick
- Off the Wall - with a spinner
- Blasteroids
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With Rampart an additional socket is required to lift the carrier board just above the 68000 that sits very tightly to the bottom of the Leta. Sockets.