Correct crystal for 80287

[aitotat]

I finally found a 80287. It's a 10 MHz part but i was wondering what crystal do i need for it. I have this motherboard with 16MHz 286 so i can't use the same crystal for 287. Should i use 20MHz crystal since x86 processors all need crystal with twice the processor clock rate?

 

[IBMMuseum]

I finally found a 80287. It's a 10 MHz part but i was wondering what crystal do i need for it. I have this motherboard with 16MHz 286 so i can't use the same crystal for 287. Should i use 20MHz crystal since x86 processors all need crystal with twice the processor clock rate?

JP1 on the motherboard might control what you want to do. In one setting it will be clocked at the same rate as the CPU (16MHz), the other at 2/3rds the CPU rate (10.666MHz, which is close enough for your 287 to tolerate). To be sure (the page doesn´t give enough details for me) you would have to see if JP1 is connected to pin 39 (Clock Mode) of the 287 socket.

You might see if pin 39 is grounded (run 287 at 2/3rds of input frequency) or tied high (run 287 at full clock input frequency) anyway...

The other arrangement (which sounds closer for this motherboard) is that the 287 is clocked separately from the 286 for one setting of JP1. I would try a 10MHz crystal first, and test the NPU speed before trying a 20MHz crystal in Y1 (does it have anything in that position now?). You won´t hurt the 287 by running it at 5MHz if the 10MHz crystal is wrong.

 

[aitotat]

I tested with multimeter that pin 39 was grounded. I tried with two external crystals and finally the same that runs the 286. Here are the results:

11.5 MHz crystal => 3.8 MHz 80287
20.0 MHz crystal => 6.7 MHz 80287
internal (32.0 MHz) crystal => 10.7 MHz 80287

So in every situation the 287 runs at 1/3 crystal clock (x86 processors run at 1/2 crystal clock).

So i didn't need any extra crystal at all. The same that runs 16 MHz 286 happened to be the correct one for 10 MHz 287.

Thank you for help. Now i finally have a working 287.