First post, by carlostex
- Rank
- l33t
Calling on all electronics experts and aficionados.
I've had this idea which is hardly new and i'm sure a lot of people back in the day designed their own.
Everyone pretty much has at least one replaced an 8088 for a NEC V20. A later version of the V20, the V20HL is capable to clock at least at 16MHz. Because all XT's run synchronously a simple crystal mod would render your motherboard useless in Turbo mode.
My idea was basically a small PCB that would have male 40pin DIP pins on the PCB bottom to place on the socket and then a female socket on top for the actual CPU. Then the PCB would include 2 crystal oscillators, a divide by three circuit and a 2 pin jumper header.
All we need is that our custom PCB responds to the Turbo button just like the system does. When the turbo header is pulled low all the system is at 4.77Mhz. When pulled high CPU and ISA CLK uses the other crystal. For instance my XT motherboard uses a 14.318 and a 30MHz crystals, hence the 4.77MHz and 10MHz speeds.
Make our PCB respond to the Turbo button the same way via the 2 pin header, it is a simple job to solder a second cable on the Turbo button, one cable for the system and other for our custom PCB. It is easier and will probably better to get the LOW/HIGH signal from the turbo cable instead of getting that signal from the motherboard. When pulled LOW use the divide by three from the 14.318 crystal. When pulled HIGH use the signal from the 48MHz crystal thus giving 4.77MHz and 16MHz respectively and exactly what we need.
Oh it would be a good idea to make the 48MHz as a socket. I guess the NEC V20HL can probably be slightly overclocked so it would be cool to have the ability to change the crystal from 48MHz to let's say 54MHz.
I sent this idea to James Pearce and i hope i didn't sound like an idiot they way i presented the idea, i'm not 100% sure this can be done this way, but if it can it's probably the best option since we don't need to mess with the motherboard in any way.
So what do the electronics experts around here think?