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FEBRUARY 2005
PRELIMINARY
XRK4991A
REV. P1.0.2
3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER
at the clock destination. This feature minimizes clock
distribution difficulty while allowing maximum system
clock speed and flexibility.
FEATURES
FUNCTIONAL DESCRIPTION
The XRK4991A 3.3V High-Speed Low-Voltage
Programmable Skew Clock Buffer offers user
selectable control over system clock functions to
optimize the timing of high-performance computer
systems. Eight individual drivers, arranged as four
pairs of user-controllable outputs, can each drive
terminated transmission lines with impedances as
low as 50Ω while delivering minimal and specified
output skews and full-swing logic levels (LVTTL).
Each output can be hardwired to one of nine delay or
function configurations. Delay increments of 0.7 to
1.5 ns are determined by the operating frequency
with outputs able to skew up to
±6
time units from
their nominal “zero” skew position. The completely
integrated PLL allows external load and transmission
line delay effects to be canceled. When this “zero
delay” capability is combined with the selectable
output skew functions, the user can create output-to-
output delays of up to
±12
time units.
Divide-by-two and divide-by-four output functions are
provided for additional flexibility in designing complex
clock systems. When combined with the internal PLL,
these divide functions allow distribution of a low-
frequency clock that can be multiplied by two or four
F
IGURE
1. B
LOCK
D
IAGRAM OF THE
XRK4991A
TEST
PE
FB_IN
PHASE
CLKIN
FSEL
SELD0
SELD1
Select Inputs
SELC0
SELC1
SELB0
SELB1
SELA0
SELA1
FREQ
DET
FILTER
•
Ref input is 5V tolerant
•
3 pairs of programmable skew outputs
•
Low skew: 200ps same pair, 250ps all outputs
•
Selectable
positive
or
negative
edge
synchronization: Excellent for DSP applications
•
Synchronous output enable
•
Output frequency: 3.75MHz to 85MHz
•
2x, 4x, 1/2, and 1/4 outputs
•
2 skew grades
•
3-level inputs for skew and PLL range control
•
PLL bypass for DC testing
•
External feedback, internal loop filter
•
12mA balanced drive outputs
•
32-pin PLCC package
•
Jitter < 200 ps peak-to-peak (< 25 ps RMS)
•
Green packaging
VCO AND TIME
UNIT GENERATOR
0E
QD0
QD1
SKEW
SELECT
QC0
QC1
QB0
MATRIX
QB1
QA0
QA1
Exar
Corporation 48720 Kato Road, Fremont CA, 94538
•
(510) 668-7000
•
FAX (510) 668-7017
•
www.exar.com
EXAR [ EXAR CORPORATION ]
