Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!interlan.InterLan.COM!interlan.interlan.com!dave From: dave@interlan.Interlan.COM (Dave Goldblatt) Newsgroups: comp.sys.ibm.pc.hardware Subject: 80287 {+ 387} clock requirements (Re: coprocessor... how fast???) Message-ID: Date: 29 Jan 91 14:14:59 GMT References: <1991Jan27.172506.15133@ncsuvx.ncsu.edu> <142@thor.UUCP> Sender: news@interlan.Interlan.COM (No News is BAD News) Reply-To: dave@interlan.interlan.com Organization: Racal InterLan, Inc., Boxborough, MA (1-800-LAN-TALK) Lines: 52 In-Reply-To: scjones@thor.UUCP's message of 28 Jan 91 19:01:42 GMT Nntp-Posting-Host: slam.interlan.com From: scjones@thor.UUCP (Larry Jones) Date: 28 Jan 91 19:01:42 GMT It must be rated for at least the clock speed that your motherboard supplies. 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 and a 16Mhz 286 would need a 12Mhz 287. Although this scheme is common, it is by no means universal, so you should really make an effort to find out the details for your particular motherboard. 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. 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 For the 80387, there are two modes of operation. The state of the CKM input determines its mode of operation, either fully synchronous, or psuedo-synchronous (only the interface logic is synchronous; the internal logic may be 10/16 to 16/10 times the speed of the CLK2 signal). Since the interface logic always runs at the same speed at the 80386, you need a 80387 which is rated at the same speed as your processor. I'm not positive about the 8087; my reference books don't go into much detail on older products. :-) It also runs on a 33% duty cycle, but I think the CPU interface needs to be clocked at the same rate as the system CPU (thus requiring an 8087 rated at the system clock speed); it doesn't support switching between using the system or an external clock. -dg- -- "Dan Rather? NOT!" * Dave Goldblatt [dave@interlan.com] - Wayne Campbell, * Diagnostic Engineering "Wayne's World" * Racal InterLan _SNL_ * Boxborough MA (508) 263-9929