modem7
10k Member
Clarification, because the wording might suggest to some that the IBM 5170 (AT) motherboard has a clock divider chip that is dividing the clock by 2/3 for the 80287. Let's look at the second 5170 motherboard as an example. The second 5170 motherboard is considered to run at 8 MHz.because the original AT boards used a 2/3 divider.
The board has a 16 MHz crystal. It feeds the 82284 clock chip. The CLK output of that chip, which is 16 MHz, is supplied to the 80286 and the 80287 (and other chips).
Within the 80286, that gets divided by 2 to create an internal clock of 8 MHz.
As for the 80287, what it does with the clock fed to it depends on whether the CKM pin (pin 39) is tied high or low. In the IBM 5170, it's tied low, which results in the 80287 dividing the clock by 3, creating an internal clock of about 5.3 MHz.
And so it is said that in the IBM 5170, the 80287 runs at two thirds (5.3 MHz / 8 MHz ) the speed of the 80286.
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