dave@interlan.Interlan.COM (Dave Goldblatt) (01/29/91)
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) 263-9929scjones@thor.UUCP (Larry Jones) (01/31/91)
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.91Jan29091459@slam.InterLan.COM>, 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