M1025 Datasheet, PDF (6/14 Pages) Integrated Circuit Systems – VCSO BASED CLOCK PLL WITH AUTOSWITCH

Integrated
Circuit
Systems, Inc.
TriState
The TriState feature puts the LVPECL output driver into
a high impedance state, effectively disconnecting the
driver from the FOUT and nFOUT pins of the device. A
logic 0 is then present on the clock net. The impedance
of the clock net is then set to 50Ω by the external circuit
resistors. (This is in distinction to a CMOS output in
TriState, in which case the net goes to a high
impedance and the logic value floats.) The 50Ω
impedance level of the LVPECL TriState allows
manufacturing In-circuit Test to drive the clock net with
an external 50Ω generator to validate the integrity of
clock net and the clock load.
Any unused output (single-ended or differential) should
be left unconnected (floating) in system application.
This minimizes output switching current and therefore
minimizes noise modulation of the VCSO.
Loss of Lock Indicator (LOL) Output Pin
Under normal device operation, when the PLL is locked,
the LOL Phase Detector drives LOL to logic 0. Under
circumstances when the VCSO cannot lock to the input
(as measured by a greater than 4 ns discrepancy
between the feedback and reference clock rising edges
at the LOL Phase Detector) the LOL output goes to logic
1. The LOL pin will return back to logic 0 when the phase
detector error is less than 2 ns. The loss of lock
indicator is a low current LVCMOS output.
M1025/26
VCSO BASED CLOCK PLL WITH AUTOSWITCH
Product Data Sheet
Guidelines Using LOL
As described, the LOL pin indicates when the PLL is
out-of-lock with the input reference. The LOL condition
is also used by the AutoSwitch circuit to detect a lost
reference, as described in following sections. LOL is
also used by the Hitless Switching and Phase Build-out
functions (optional device features).
To ensure reliable operation of LOL and guard against
false out-of-lock indications, the following conditions
should be met:
• The phase detector frequency should be no less than
5MHz, and preferably it should be 10MHz or greater.
Phase detector frequency is defined by Fin / R.
A higher phase detector frequency will result in lower
phase error and less chance of false triggering the
LOL phase detector. Refer to Tables 3 and 4 on pg. 3
for phase detector frequency when using the
M1025-11-155.5200 or the M1026-11-155.5200.
• The input reference should have an intrinsic jitter of
less than 1 ns pk-pk. If reference jitter is greater than
1 ns pk-pk, the LOL circuit might falsely trigger. Due
to this limitation, the LOL circuit should not be used in
loop timing mode, nor should it be used with a noisy
reference clock. Likewise, the AutoSwitch, Hitless
Switching, or Phase Build-out features should not be
used in loop timing mode or with a noisy reference
clock, since these features depend on LOL.
Reference Acknowledgement (REF_ACK) Output
The REF_ACK (reference acknowledgement) pin outputs
the value of the reference clock input that is routed to
the phase detector. Logic 1 indicates input pair 1
(nDIF_REF1, DIF_REF1); logic 0 indicates input pair 0
(nDIF_REF0, DIF_REF0). The REF_ACK indicator is an
LVCMOS output.
M1025/26 Datasheet Rev 1.0
6 of 14
Revised 28Jul2004
Integrated Circuit Systems, Inc. ● Networking & Communications ● www.icst.com ● tel [phone]