Compaq 21264 - Manual (Page 213)

Alpha 21264/EV67 Hardware Reference Manual Initialization and Configuration 7–5
Power-Up Reset Flow and the Reset_L and DCOK_H Pins
Table 7–3 summarizes the pins and the suggested/required initialization state. Most of
this information is supplied by placing (switch-selectable or hardwired) weak pull-ups
or pull-downs on the IRQ_H pins. The IRQ_H pins are sampled on the rising edge of
DCOK_H, during which time the 21264/EV67 is in reset and is not generating any sys-
tem activity. During normal operation, the IRQ_H pins supply interrupt requests to the
21264/EV67.
It is possible to disable the 21264/EV67 PLL and source GCLK directly from ClkIn_x.
This mode is selected via PllBypass_H. The 21264/EV67 still produces a divided-
down clock on EV6Clk_x; this output clock, which tracks GCLK, can be used in a
feedback loop to generate a locked input clock via an external PLL. The input clock
can be locked against a slower speed system reference clock.
Table 7–3 Pin Signal Names and Initialization State
Signal Name Sample Time Function Value
PllBypass_H Continuous input Select ClkIn_x onto GCLK instead of internal
PLL.
0 Bypass
1
1 Use PLL
ClkFwdRst_H Sampling method
according to
IRQ_H[4]
— —
Reset_L Continuous input — —
IRQ_H[5] Rising edge of
DCOK_H Select 1:1 FrameClk mode.
Internal FrameClk can be generated two ways: 0 Sample with
FrameClk_H
1 Use a copy of
EV6Clk_H
IRQ_H[4] Rising edge of
DCOK_H Select method of sampling ClkFwdRst_H to
produce internal ClkFwdRst — either with
external or internal copy of FrameClk_x. 0 Sample with Exter-
nal FrameClk_x
1 Sample with Inter-
nal Frameclk
IRQ_H[3:0] Rising edge of
DCOK_H Select Y
div
divisor value. This is the divide-
down factor between GCLK and EV6Clk_x.
When the PLL is in use and the 21264/EV67 is
ramped-up to full speed, the VCO adjusts in
order to phase-align (and rate-match) EV6Clk_x
to ClkIn_x. When the PLL is not in use, and
ClkIn_x is bypassed onto GCLK, EV6Clk_x is
slower than ClkIn_x by the divisor Y
div
.
IRQ_H[3:0] Divisor
0011 3
0100 4
0101 5
0110 6
0111 7
0000 8
1000 9
1001 10
1010 11
1011 12
1100 13
1101 14
1110 15
1111 16
1 By sampling FrameClk_H. Used if
FrameClk_H is slower than ClkIn_H.
2 As a direct copy of EV6Clk_H. Used if
FrameClk_H is the same frequency as
ClkIn_H or is DC.