Started by gborgns, January 12, 2006, 07:36:11 PM
Quote from: "gborgns"Almost the entire processing portion of the computer is accelerated to a base clock of 50MHz, including the memory and SCSI interfaces. This requires the use of faster memory and more modern SCSI drives.
Quote from: "gborgns"No, the stock Quaddoubler works only with an 040 Mac and used a PAL to sense when a bus relinquish signal was active. The PAL allowed a doubled clock signal to clock the processor during the bus relinquish periods. This allowed the processor to run at a faster rate than the system bus. This signal is not used in the NeXT computer. The bus is never released. The entire computer must be synchronized to the new clock frequencies. The NeXT is a very different animal than the MAC.
Quote from: "gborgns"The 040 requires two clocks, one at twice the base clock frequency. So a 33MHz 040 also has a 66MHz clock applied to it which is provided by the 70MHz rated MC88915. So yes, the maximum clock out of the new MC88915 is 100MHz (it actually has a higher frequency signal available, but it is not used), but that is what a 50MHz 040 requires, 50MHz and 100MHz.
Quote from: "gborgns"I hope someone else will try this mod and take it even further.
Quote from: "gborgns"A8 ) I used 60ns EDO RAM. I didn't try slower memory.
Quote from: "gborgns"A9) I didn't fully analyze the Quaddoubler PAL. When I found that it did not work in it's stock form in the NeXT, I started to look at what signals it was monitoring. The Bus request, Bus Grant and Bus Busy signals stood out. The Bus Busy line is pulled up and is inactive on the NeXT. The doubler was somehow using the Bus Busy line on a MAC to know when it could clock the 040 at a higher rate. That was the extent on my analysis.
Quote from: "gborgns"A10) The 80MHz MC88916 on the homemade clock doubler, doubles the 25MHz system clock to 50MHz. That 50MHz clock is then feed into the 100MHz MC88915 to create the synchronized 50/100MHz clocks for the overclocked 040. The 100MHz MC88915 is on the system board and replaced the stock 80MHz part. The 50MHz clock is also fed into the rest of the NeXT circuitry (CLKBCLK) to keep everything synchronized.
Quote from: "gborgns"I'll have to power up my system again to see, but I am pretty sure the memory actual showed up as 60ns on boot.
Quote from: "gborgns"I didn't so much as piggy-back the doublers, as ran them in series to get from 25MHz to 50MHz, then 50MHz to 100Mhz.U45, the MC88915, is right next to the processor and is under the floppy drive on the slab. The clock chip next to the lithium battery is the real time clock.
Quote from: "gborgns"Correct, except that the 25MHz originates from the TMC chip which uses the 100MHz buffered video clock (CLK100MTTLN) as an input to derive the 25MHz clock. See the Cslab schematics for more info.