Posted By
TLC
on 2017-09-10
09:26:56
|  Re: Is possible to create PAL/NTSC TED software switch?
You'll need to decide if you want the video standard to be switched "on the fly" or statically.
Either way, you need clocks for both PAL and NTSC modes.
For a static switch, it's O.K. if you hack a second crystal + trimmer capacitor somewhere around the original one, and use a switch to select either of them. Note that 1.) it won't be possible to switch clocks on the fly, since the crystals need some time to start oscillating and stabilize (...inbetween, there'd be no clock), which typically crashes the machine. And 2.) the machine will boot up from the original Kernal ROM regardless to the setting of the switch. To fix that, you probably also need to hack in a 27256 eprom or some flash with both a PAL and NTSC rom images, and select the active Kernal image according to the PAL/NTSC switch.
For an on-the-fly hack you probably need a pair of oscillator modules at the first place (so that you have both clocks available all the time, without transients), and figure out how to make the machine toggle $ff07 bit 6 (the TED PAL/NTSC bit) upon toggling the switch. But this all looks reasonably more difficult.
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