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XRK4991A Datasheet(PDF) 5 Page - Exar Corporation

Part No. XRK4991A
Description  3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER
Download  13 Pages
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Maker  EXAR [Exar Corporation]
Homepage  http://www.exar.com
Logo EXAR - Exar Corporation

XRK4991A Datasheet(HTML) 5 Page - Exar Corporation

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PRELIMINARY
XRK4991A
REV. P1.0.2
3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER
5
SKEW SELECT MATRIX
The skew select matrix is comprised of four independent sections. Each section has two low-skew, high-fanout
drivers (Qx[0:1]), and two corresponding three-level function select (SELx[0:1]) inputs. Table 2 below shows
the nine possible output functions for each section as determined by the function select inputs. All times are
measured with respect to the CLKIN input assuming that the output connected to the FB_IN input has 0tU
selected.
NOTES:
1. For all three-state inputs, HIGH indicates a connection to VCC, LOW indicates a connection to GND, and MID
indicates an open connection. Internal termination circuitry holds an unconnected input to VCC/2.
2. The level to be set on FSEL is determined by the “normal” operating frequency (fNOM) of the VCO and Time Unit
Generator (see Logic Block Diagram). Nominal frequency (fNOM) always appears at QA0 and the other outputs
when they are operated in their undivided modes (see Table 2). The frequency appearing at the CLKIN and FB_IN
inputs will be fNOM when the output connected to FB_IN is undivided. The frequency of the CLKIN and FB_IN
inputs will be fNOM/2 or fNOM/4 when the part is configured for a frequency multiplication by using a divided output
as the FB_IN input.
3. When the FSEL pin is selected HIGH, the CLKIN input must not transition upon power-up until VCC has reached
2.8V.
TABLE 2: FREQUENCY RANGE SELECT AND tU CALCULATION [1]
fNOM (MHZ)
tU = 1 / fNOM X N
APPROXIMATE
FREQUENCY (MHZ) AT
WHICH
tU = 1.0ns
FSEL[2,3]
MIN
MAX
WHERE
N =
LOW
15
30
44
22.7
MID
25
50
26
38.5
HIGH
40
85
16
62.5
TABLE 3: PROGRAMMABLE SKEW CONFIGURATIONS [1]
FUNCTION SELECTS
OUTPUT FUNCTIONS
SELX1
SELX0
QA[1:0], QB[1:0]
QC[1:0]
QD[1:0]
LOW
LOW
-4tU
Divide by 2
Divide by 2
LOW
MID
-3tU
-6tU
-6tU
LOW
HIGH
-2tU
-4tU
-4tU
MID
LOW
-1tU
-2tU
-2tU
MID
MID
0tU
0tU
0tU
MID
HIGH
+1tU
+2tU
+2tU
HIGH
LOW
+2tU
+4tU
+4tU
HIGH
MID
+3tU
+6tU
+6tU
HIGH
HIGH
+4tU
Divide by 4
Inverted


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