Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!sdrc!thor!scjones From: scjones@thor.UUCP (Larry Jones) Newsgroups: comp.sys.ibm.pc.hardware Subject: Re: 80287 {+ 387} clock requirements (Re: coprocessor... how fast???) Message-ID: <150@thor.UUCP> Date: 30 Jan 91 23:27:36 GMT References: <1991Jan27.172506.15133@ncsuvx.ncsu.edu> <142@thor.UUCP> Organization: SDRC, Cincinnati Lines: 40 I said: > In most cases, the external clock supplied to the 287 is > the same as the external clock supplied to the 286 -- the 286 divides > the external clock by 2 to get the internal clock (which is what the > rating refers to) but the 287 divides the external clock by 3, so the > 287 would need to be at least 2/3 the speed of the 286. Thus, a > 12MHz 286 would need an 8Mhz 287 ... In article , dave@interlan.Interlan.COM (Dave Goldblatt) writes: > Not quite. The 80287 does indeed divide the system clock by three when > the CKM line is low. But since it is running on a 33% duty cycle, > you only need a 287 rated at 1/3 of the system clock speed. Yes, quite. You misread what I wrote since what you said is exactly what I said. The 286 divides the system clock by 2, so the 287 should be rated for 1/3 the system clock speed which is identical to 2/3 the rated speed of the 286. So a 12 MHz 286 would have a 24 MHz system clock and thus require an 8 MHz 287 just like we both said. > From the > Intel _Microprocessor and Peripheral Handbook_, volume I, 1989: > > 287 Speed CLK Speed > Version CKM = 0 CKM = 1 > --------- ------- ------- > 5 MHz 12 MHz 5 MHz > 6 MHz 16 MHz 6 MHz > 8 MHz 20 MHz 8 MHz > 10 MHz 25 MHz 10 MHz I've never understood this chart -- the "CKM = 0" column should be "15 MHz, 18 MHz, 24 MHz, and 30 MHz" and I don't know why it's not. My only guess is that it actually shows the 287 required for common clock rates rather than giving the maximum clock rate for each 287 as it appears to. ---- Larry Jones, SDRC, 2000 Eastman Dr., Milford, OH 45150-2789 513-576-2070 Domain: scjones@thor.UUCP Path: uunet!sdrc!thor!scjones I hate being good. -- Calvin