Thanks to the efforts of foft (Mark Watson) and his Atari FPGA project, there is an open source FPGA pokey implementation available. As a little Easter bank holiday project, I thought I'd have a go at using it to make a dual pokey cartridge using a prototype of the Ultimate Cart as the starting point...
Two of the fpga pins act as audio left and right, and I've hooked them up to a low pass filter on some perfboard, along with a 3.5mm stereo jack (which leads to my TV's audio input).
It was pretty easy to make a firmware for the Ultimate Cart that included two of foft's pokeys, his DAC and a bit of VHDL to hook them up to the cartridge port.
Then I modified TMC and RMTPlayer (with a hex editor) to output to the two pokeys at $D50x and $D58x.
I guess technically this atari has 3 pokeys now (one inside, and two on the cartridge). The two pokeys use about 10% of the Ultimate Cart's FPGA, so there is plenty room for more.
This was just for fun, and perhaps to show a little of what is possible with a FPGA hooked up to the cart port. To make a proper external dual pokey, we'd probably want to use ECI+Cart port, so the pokeys could appear in the correct places in the memory map.
MP3 of TMC playing via the dual pokeys attached.
Edited by electrotrains, Fri Apr 1, 2016 6:28 AM.