4
INDUSTRIAL TEMPERATURE RANGE
IDT5T9955
2.5V PROGRAMMABLE SKEW DUAL PLL CLOCK DRIVER TURBOCLOCK W
Output skew with respect to the REF input is adjustable to compensate
for PCB trace delays, backplane propagation delays or to accommodate
requirements for special timing relationships between clocked compo-
nents. Skew is selectable as a multiple of a time unit (tU) which ranges
from 782ps to 1.5625ns (see Programmable Skew Range and Resolu-
tion Table). There are nine skew configurations available for each out-
put pair. These configurations are chosen by the xnF
1:0 control pins. In
order to minimize the number of control pins, 3-level inputs (HIGH-MID-
LOW) are used, they are intended for but not restricted to hard-wiring.
Undriven 3-level inputs default to the MID level. Where programmable
skew is not a requirement, the control pins can be left open for the zero
skew default setting. The Control Summary Table shows how to select
specific skew taps by using the xnF1:0 control pins.
PROGRAMMABLE SKEW
EXTERNAL FEEDBACK
By providing two separate external feedbacks, the IDT5T9955 gives
users flexibility with regard to skew adjustment. The xFB signal is com-
pared with the input REF signal at the phase detector in order to drive
the VCO. Phase differences cause the VCO of the PLL to adjust up-
wards or downwards accordingly.
NOTES:
1. The device may be operated outside recommended frequency ranges without damage, but functional operation is not guaranteed.
2. The level to be set on xFS is determined by the nominal operating frequency of the VCO and Time Unit Generator. The VCO frequency always appears at x1Q1:0, x2Q1:0, and
the higher outputs when they are operated in their undivided modes. The frequency appearing at the REF and xFB inputs will be FNOM when the output connected to xFB is
undivided and xDS[1:0] = MM. The frequency of the REF and xFB inputs will be FNOM /2 or FNOM /4 when the part is configured for frequency multiplication by using a divided
output as the xFB input and setting xDS[1:0] = MM. Using the xDS[1:0] inputs allows a different method for frequency multiplication (see Divide Selection Table).
3. Skew adjustment range assumes that a zero skew output is used for feedback. If a skewed xQ output is used for feedback, then adjustment range will be greater. For example
if a 4tU skewed output is used for feedback, all other outputs will be skewed –4tU in addition to whatever skew value is programmed for those outputs. ‘Max adjustment’ range
applies to output pairs 3 and 4 where ±6tU skew adjustment is possible and at the lowest FNOM value.
An internal loop filter moderates the response of the VCO to the
phase detector. The loop filter transfer function has been chosen to
provide minimal jitter (or frequency variation) while still providing accu-
rate responses to input frequency changes.
xFS = LOW xFS = MID xFS = HIGH Comments
Timing Unit Calculation (tU) 1/(32 x FNOM) 1/(16 x FNOM) 1/(8 x FNOM)
VCO Frequency Range (FNOM)
(1,2)
24 to 40MHz 40 to 80MHz 80 to 160MHz
Skew Adjustment Range
(3)
Max Adjustment: ±7.8125ns ±9.375ns ±9.375ns ns
±67.5° ±135° ±270° Phase Degrees
±18.75% ±37.5% ±75% % of Cycle Time
Example 1, FNOM = 25MHz tU = 1.25ns — —
Example 2, FNOM = 37.5MHz tU = 0.833ns — —
Example 3, FNOM = 50MHz — tU = 1.25ns —
Example 4, FNOM = 75MHz — tU = 0.833ns —
Example 5, FNOM = 100MHz — — tU = 1.25ns
Example 6, FNOM = 150MHz — — tU = 0.833ns
PROGRAMMABLE SKEW RANGE AND RESOLUTION TABLE
