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XRK4991A Datasheet(PDF) 5 Page - Exar Corporation |
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XRK4991A Datasheet(HTML) 5 Page - Exar Corporation |
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5 / 13 page 
xr 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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