Z9972
Document #: 38-07088 Rev. *D
Page 3 of 9
Description
The Z9972 has an integrated PLL that provides low-skew and
low-jitter clock outputs for high-performance microprocessors.
Three independent banks of four outputs as well as an
independent PLL feedback output, FB_OUT, provide excep-
tional flexibility for possible output configurations. The PLL is
ensured stable operation given that the VCO is configured to
run between 200 MHz to 480 MHz. This allows a wide range
of output frequencies up to125 MHz.
The phase detector compares the input reference clock to the
external feedback input. For normal operation, the external
feedback input, FB_IN, is connected to the feedback output,
FB_OUT. The internal VCO is running at multiples of the input
reference clock set by FB_SEL(0:2) and VCO_SEL select
inputs (see Table 1). The VCO frequency is then divided to
provide the required output frequencies. These dividers are
set by SELA(0,1), SELB(0,1), SELC(0,1) select inputs (see
Table 2). For situations in which the VCO needs to run at
relatively low frequencies and therefore might not be stable,
assert VCO_SEL LOW to divide the VCO frequency by 2. This
will maintain the desired output relationships, but will provide
an enhanced PLL lock range.
The Z9972 is also capable of providing inverted output clocks.
When INV_CLK is asserted HIGH, QC2 and QC3 output
clocks are inverted. These clocks could be used as feedback
outputs to the Z9972 or a second PLL device to generate early
or late clocks for a specific design. This inversion does not
affect the output-to-output skew.
Glitch-Free Output Frequency Transitions
Customarily when output buffers have their internal counter’s
changed “on the fly’ their output clock periods will:
1. contain short or “runt” clock periods. These are clock cycles
in which the cycle(s) are shorter in period than either the
old or new frequency to which they are being transitioned.
2. contain stretched clock periods. These are clock cycles in
which the cycle(s) are longer in period than either the old
or new frequency to which they are being transitioned.
This device specifically includes logic to guarantee that runt
and stretched clock pulses do not occur if the device logic
levels of any or all of the following pins changed “on the fly”
while it is operating: SELA, SELB, SELC, and VCO_SEL.
SYNC Output[3]
In situations where output frequency relationships are not
integer multiples of each other the SYNC output provides a
signal for system synchronization. The Z9972 monitors the
relationship between the QA and the QC output clocks. It
provides a low going pulse, one period in duration, one period
prior to the coincident rising edges of the QA and QC outputs.
The duration and the placement of the pulse depend on the
higher of the QA and QC output frequencies. The following
timing diagram illustrates various waveforms for the SYNC
output.
Note:.
3.
The SYNC output is defined for all possible combinations of the QA and QC outputs even though under some relationships the lower frequency clock could be
used as a synchronizing signal.
Table 2. Frequency Select Inputs
VCO_SEL
SELA1
SELA0
QA
SELB1
SELB0
QB
SELC1
SELC0
QC
0
0
0
VCO/8
0
0
VCO/8
0
0
VCO/4
0
0
1
VCO/12
0
1
VCO/12
0
1
VCO/8
0
1
0
VCO/16
1
0
VCO/16
1
0
VCO/12
0
1
1
VCO/24
1
1
VCO/20
1
1
VCO/16
1
0
0
VCO/4
0
0
VCO/4
0
0
VCO/2
1
0
1
VCO/6
0
1
VCO/6
0
1
VCO/4
1
1
0
VCO/8
1
0
VCO/8
1
0
VCO/6
1
1
1
VCO/12
1
1
VCO/10
1
1
VCO/8
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