TimWolf wrote on 2020-05-04, 20:46:

I know that the i287-XL was intended to be 12.5mhz but many have said it can be pushed further as it is really a 387. It is my understanding that the clock will either be half or 66% of the original, but I'm not sure with board.

NPU clock setting depends on CKM pin (#39) - if it's a logic level L then the clock will be divided by 3 internally. If it's at H then clock will not be divided but used as-is.
Typically 286 mobos work in synchronous mode - that means /3 divider setting. Keep in mind the clock referenced here is the CPU input clock, so the actual crystal frequency, not the cycle clock. Which would give you 50MHz/3=16.6(6)MHz

In asynchronous mode the mobo can clock the NPU without the divider but that means CPU and NPU clocks are now possibly different and maybe not even in phase, so there must be some extra glue logic between the two. Because of that it's rare for any mobo to implement this, it's only useful for those mobos that used 20+ MHz CPUs so the NPU would have to be run slower as there were no suitable parts on the market for these speeds. Or, you know, wouldn't support NPU at all.

Oh, and there's a catch - assuming it's not a typo in the 287XL datasheet. It uses /2, not /3 divider, so you'd better get the fastest rated part you can 😀

Deunan wrote on 2020-05-04, 22:22:

NPU clock setting depends on CKM pin (#39) - if it's a logic level L then the clock will be divided by 3 internally. If it's at […]
Show full quote

TimWolf wrote on 2020-05-04, 20:46:

I know that the i287-XL was intended to be 12.5mhz but many have said it can be pushed further as it is really a 387. It is my understanding that the clock will either be half or 66% of the original, but I'm not sure with board.

NPU clock setting depends on CKM pin (#39) - if it's a logic level L then the clock will be divided by 3 internally. If it's at H then clock will not be divided but used as-is.
Typically 286 mobos work in synchronous mode - that means /3 divider setting. Keep in mind the clock referenced here is the CPU input clock, so the actual crystal frequency, not the cycle clock. Which would give you 50MHz/3=16.6(6)MHz

In asynchronous mode the mobo can clock the NPU without the divider but that means CPU and NPU clocks are now possibly different and maybe not even in phase, so there must be some extra glue logic between the two. Because of that it's rare for any mobo to implement this, it's only useful for those mobos that used 20+ MHz CPUs so the NPU would have to be run slower as there were no suitable parts on the market for these speeds. Or, you know, wouldn't support NPU at all.

Oh, and there's a catch - assuming it's not a typo in the 287XL datasheet. It uses /2, not /3 divider, so you'd better get the fastest rated part you can 😀

Here is the intended 287xl.

Deunan wrote on 2020-05-04, 22:22:

NPU clock setting depends on CKM pin (#39) - if it's a logic level L then the clock will be divided by 3 internally. If it's at […]
Show full quote

TimWolf wrote on 2020-05-04, 20:46:

I know that the i287-XL was intended to be 12.5mhz but many have said it can be pushed further as it is really a 387. It is my understanding that the clock will either be half or 66% of the original, but I'm not sure with board.

NPU clock setting depends on CKM pin (#39) - if it's a logic level L then the clock will be divided by 3 internally. If it's at H then clock will not be divided but used as-is.
Typically 286 mobos work in synchronous mode - that means /3 divider setting. Keep in mind the clock referenced here is the CPU input clock, so the actual crystal frequency, not the cycle clock. Which would give you 50MHz/3=16.6(6)MHz

In asynchronous mode the mobo can clock the NPU without the divider but that means CPU and NPU clocks are now possibly different and maybe not even in phase, so there must be some extra glue logic between the two. Because of that it's rare for any mobo to implement this, it's only useful for those mobos that used 20+ MHz CPUs so the NPU would have to be run slower as there were no suitable parts on the market for these speeds. Or, you know, wouldn't support NPU at all.

Oh, and there's a catch - assuming it's not a typo in the 287XL datasheet. It uses /2, not /3 divider, so you'd better get the fastest rated part you can 😀

Letting the 287 take a back seat for a moment, I have the socket for the crystal in the motherboard to upgrade the 286 cpu. I've ripped out the cpu, and put in a PLCC68 socket and added the 25mhz chip. With the 50.00mhz crystal the new 25mhz chip will not boot. Put the 24.00mhz crystal back in it does boot and work perfectly at the original 12mhz, and clocks down to 8 with the turbo button on. The bios is an Ami Bios 286 plus from 1991. Any tips or suggestions for getting the clock to work with the cpu? There is an unpopulated crystal location and I think that may be something to do with it. Could any electronic gurus look at that and determine if they can figure out what is going on here?

Thanks in advance,
TW

TimWolf wrote on 2020-05-12, 22:37:

With the 50.00mhz crystal the new 25mhz chip will not boot. Put the 24.00mhz crystal back in it does boot and work perfectly at the original 12mhz

I figured you were already aware so I didn't mention it but quite likely the chipset is not fast enough to work at 50MHz. Another possibility is the wait states for BIOS ROM and RAM, as well some of the I/O chips. Those WS could be hardwired for a slower clock. Speaking of RAM, older mobos use a delay line rather than implementing waitstates in ASIC logic - I don't see one on yours but the photos are not showing some part of the mobo. That too might be too slow (and can't be made faster, it needs to properly sequence signals to RAM). It all depends on the mobo design.

In general you can't just replace the CPU clock generator and expect a 286 mobo to just work unless it was already made for faster speeds. That being said perhaps you can make it work with some in-between value. Try 32 and 40 MHz clocks.

The following eBay listing is the same board.

https://www.ebay.com/itm/Motherboard-with-N80 … rs/254530094154

I am going to try the suggestion of 32 and 40mhz crystals, because even 16 or 20mhz would be a victory in my quest.

Thanks

Okay, tried it out with a 35.xx mhz crystal I harvested from an old broken router. It will boot up and post but the hdd (ide onboard) will not load up. It works fine with the original 24mhz crystal. I think this confirms that the crystal is being shared and clocked down somewhere. The question now is, can it be caught down the line and clocked down further for the aspects of the board that need it?

Check the BIOS. Most 286 only allow running the bus with either full speed or 1/2 speed. With some luck, there's an 1/3 option.

It is my understanding that the clock will either be half or 66% of the original, but I'm not sure with board.

A 287 is driven with 2/3 of the CPU speed, unless you install a separate crystal for it. The XL however increases that internally back to full speed, so it runs at the same speed as the CPU. This is bad for overclocking... It seems there's an option to install a crystal for the FPU, however.

Oops never mind Deunan already mentioned it.

We have an acceptable victory on this! After removing the on board base memory 640k in bank 0, and adding four 1mb sticks, we post 3,712mb. In addition to this I put in an ET4000 video card. It worked stable with the 35mhz for hours, and so I tried the 40mhz, and it works great as well. I don't know why, but every now and then the 287 clocks down to 4mhz, but both times, I was messing with the turbo button on bootup, and a reset fixed the issue. I'm getting the full 20mhz. This has been a long time poking at the project, but a win!

TimWolf wrote on 2021-01-13, 19:52:

We have an acceptable victory on this! After removing the on board base memory 640k in bank 0, and adding four 1mb sticks, we post 3,712mb. In addition to this I put in an ET4000 video card. It worked stable with the 35mhz for hours, and so I tried the 40mhz, and it works great as well. I don't know why, but every now and then the 287 clocks down to 4mhz, but both times, I was messing with the turbo button on bootup, and a reset fixed the issue. I'm getting the full 20mhz. This has been a long time poking at the project, but a win!

Good Job ! Glad you got it working OK.. Great little 286 !