Compaq 21264 - Manual (Page 228)

7–20 Initialization and Configuration Alpha 21264/EV67 Hardware Reference Manual
Phase-Lock Loop (PLL) Functional Description
Table 7–12 shows the allowable ClkIn_x frequencies for a given operating frequency
of the 21264/EV67 and the Y
div
divider. For example, to set the 21264/EV67 GCLK
frequency to 500 MHz with a ClkIn_x frequency of 166.7 MHz, the system must select
a Y
div
divider of 3 by placing the value 0011
2
on pins IRQ_H[3:0].
Table 7–12 Differential Reference Clock Frequencies in Full-Speed Lock
7.11.2.4 Power-Up/Reset Clocking
During the power-up/reset sequence, when not in PLL bypass mode, there may be a
period of time when ClkIn_x is not yet running, but there is a voltage on PLL_VDD.
The signal DCOK_H is deasserted until power is good throughout the system. The
10% to 90% rise time of DCOK_H should be less than 2 ns. The deasserted state of
DCOK_H and the presence of PLL_VDD causes the PLL to generate a global clock
that is distributed throughout the 21264/EV67 with a frequency range of 1 MHz to 500
MHz. The presence of the global clock during this period avoids permanent damage to
the 21264/EV67.
GCLK Reference Clock Frequency (MHz) for Y
div
Dividers
1
1
Dividers 11 through 16 are out of range for the 21264/EV67 and reserved for future use. Valid refer-
ence clock (ClkIn_x) frequencies for the 21264/EV67 are specified in the range from 80 to 200.
Divider values that are out of that range are displayed as a dash “—”.
Period (ns) Frequency (MHz) 3
2
2
Dividers of 1 and 2 are to be used only in a PLL test mode.
4 5 6 7 8 9 10 11
2.5 400 133.3 100 80 — — — — — —
2.4 416.7 138.9 104.2 83.3 — — — — — —
2.3 434.8 144.9 108.7 87.0 — — — — — —
2.2 454.5 151.2 113.6 90.9 — — — — — —
2.1 476.2 158.7 119.0 95.2 — — — — — —
2.0 500 166.7 125.0 100 83.3 — — — — —
1.9 526.3 175.4 131.6 105.3 87.7 — — — — —
1.8 555.6 185.2 138.9 111.1 92.6 — — — — —
1.7 588.2 196.1 147.1 117.6 98.0 84.0 — — — —
1.6 625 — 156.3 125.0 104.2 89.3 — — — —
1.5 666.7 — 166.7 133.3 111.1 95.2 83.3 — — —
1.4 714.3 — 178.6 142.9 119.1 102.0 89.3 — — —
1.3 769.2 — 192.3 153.8 128.2 109.9 96.2 85.5 — —
1.2 833.3 — — 166.7 138.9 119.0 104.2 92.6 83.3 —