XRK4991 Datasheet, PDF (4/12 Pages) Exar Corporation – 3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER

XRK4991
3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER
xr
REV. 1.0.1
FSEL^[2]
LOW
MID
HIGH^[3]
TABLE 1: FREQUENCY RANGE SELECT AND t_U CALCULATION ^[1]
f_NOM (MHZ)
t_U = 1 / (f_NOM X N)
MIN
MAX
WHERE N =
15
30
44
25
50
26
40
85
16
APPROXIMATE
FREQUENCY (MHZ) AT
WHICH t_U = 1.0ns
22.7
38.5
62.5
SKEW SELECT CONTROL
The skew select control consists of four independent banks. Each bank has two low-skew, high-fanout drivers
(Qx0, Qx1), and two corresponding tri-level function select (SELx0, SELx1) inputs. The nine possible output
states for each bank are shown in Table 2 as determined by each bank’s select inputs. All timing
measurements are made with respect to the CLKIN input with the output connected to the FB_IN input
configured for 0 t_U operation.
TABLE 2: PROGRAMMABLE SKEW CONFIGURATIONS ^[1]
FUNCTION SELECT INPUTS
SELX1
SELX0
LOW
LOW
LOW
MID
LOW
HIGH
MID
LOW
MID
MID
MID
HIGH
HIGH
LOW
HIGH
MID
HIGH
HIGH
QA[1:0], QB[1:0]
-4t_U
-3t_U
-2t_U
-1t_U
0t_U
+1t_U
+2t_U
+3t_U
+4t_U
OUTPUT FUNCTIONS
QC[1:0]
÷2
-6t_U
-4t_U
-2t_U
0t_U
+2t_U
+4t_U
+6t_U
÷4
QD[1:0]
÷2
-6t_U
-4t_U
-2t_U
0t_U
+2t_U
+4t_U
+6t_U
Inverted
NOTES:
1. For all tri-level (three-state) inputs, HIGH indicates a connection to V_CC, LOW indicates a connection to GND, and
MID indicates an open connection. Internal termination circuitry holds an unconnected input to V_CC/2.
2. The level to be set on FSEL is determined by the “normal” operating frequency (f_NOM) of the PLL. Nominal
frequency (f_NOM) 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 f_NOM when the output connected to
FB_IN is undivided. The frequency of the CLKIN and FB_IN inputs will be f_NOM ÷ 2 or f_NOM ÷ 4 when the part is
configured for a frequency multiplication.
3. When the FSEL pin is selected HIGH, the CLKIN input must not transition upon power-up until V_CC has reached
2.8V.
4