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MARCH 2005
PRELIMINARY
XRK7988
REV. P1.0.1
INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER
device will switch to the good secondary clock and phase/frequency alignment will occur with minimal output phase disturbance. The typical phase bump caused by a failed clock is eliminated. FEATURES
GENERAL DESCRIPTION
The XRK7988 is a PLL clock driver designed specifically for redundant clock tree designs. The device receives two differential LVPECL clock signals from which it generates 5 new differential LVPECL clock outputs. Two of the output pairs regenerate the input signals frequency and phase while the other three pairs generate 8x, phase aligned clock outputs. External PLL feedback is used to also provide zero delay buffer performance. The XRK7988 Intelligent Dynamic Clock Switch circuit continuously monitors both input CLK signals. Upon detection of a failure (CLK stuck HIGH or LOW for at least 1 period), the INP_BAD for that CLK will be latched (H). If that CLK is the primary clock, the FIGURE 1. BLOCK DIAGRAM OF THE XRK7988
* Fully Integrated PLL * Intelligent Dynamic Clock Switch * LVPECL Clock Outputs * LVCMOS Control I/O * 3.3V Operation * 32-Lead LQFP Packaging * 19.44 to 155.52 MHz
CLK_Selected INP1Bad INP0Bad Man_Override Alarm_Reset Sel_CLK
Dynamic Switch Logic
PLL_En Qb0 Qb0 Qb1 Qb1 /2 Qb2 Qb2 Qa0 Qa0 Qa1 Qa1
OR
CLK0 CLK0 CLK1 CLK1 Ext_FB Ext_FB MR
PLL 200-400MHz
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Exar Corporation 48720 Kato Road, Fremont CA, 94538 * (510) 668-7000 * FAX (510) 668-7017 * www.exar.com
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REV. P1.0.1
PRELIMINARY XRK7988 INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER
PRODUCT ORDERING INFORMATION
PRODUCT NUMBER XRK7988 PACKAGE TYPE 32-Lead LQFP OPERATING TEMPERATURE RANGE -40C to +85C
FIGURE 2. PIN OUT OF THE XRK7988
VCC
24
23
22
21
20
19
18
Qa1 Qa1 Qa0 Qa0 VCC VCC_PLL Man_Override PLL_En
17
VCC
Qb0
Qb0
Qb1
Qb1
Qb2
Qb2
25 26 27 28 29 30 31 32 1 2 3 4 5 6 7 8
16 15 14
VCC Inp0bad Inp1bad CK_Selected GND Ext_FB Ext_FB GND
XRK7988
13 12 11 10 9
Alarm_Reset
Sel_CLK
CLK0
CLK0
CLK1
2
CLK1
GND
MR
XRK7988 PRELIMINARY INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER
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PIN DESCRIPTIONS
PIN NAME CLK0, CLK0 CLK1, CLK1 Ext_FB, Ext_FB Qa[1:0], Qa[1:0] Qb[2:0], Qb[2:0] Inp0bad TYPE LVPECL Input LVPECL Input LVPECL Input LVPECL Output LVPECL Output LVCMOS Output DESCRIPTION Differential PLL clock reference (CLK0 pulldown, CLK0 pullup) Differential PLL clock reference (CLK1 pulldown, CLK1 pullup) Differential PLL feedback clock (Ext_FB pulldown, Ext_FB pullup) Differential 1x output pairs Differential 8x output pairs Indicates detection of a bad input reference clock 0 with respect to the feedback signal. The output is active HIGH and will remain HIGH until the alarm reset is asserted. Indicates detection of a bad input reference clock 1 with respect to the feedback signal. The output is active HIGH and will remain HIGH until the alarm reset is asserted. 0 - if clock 0 is selected 1 - if clock 1 is selected 0 - will reset the input bad flags and align Clk_Selected with Sel_Clk. The input is one-shotted (50K pullup). 0 - selects CLK0 1 - selects CLK1 (50k pulldown) 1 - disables internal clock switch circuitry (50K pulldown). 0 - bypasses selected input reference around the phase-locked loop (50K pullup). 0 - resets the internal dividers forcing Q outputs LOW. Asynchronous to the clock (50K pullup). PLL power supply Digital power supply PLL Ground Digital Ground
Inp1bad
LVCMOS Output
Clk_Selected Alarm_Reset Sel_Clk Manual_Override PLL_En MR VCCA VCC GNDA GND
LVCMOS Output LVCMOS Input LVCMOS Input LVCMOS Input LVCMOS Input LVCMOS Input Power Supply Power Supply Power Supply Power Supply
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PRELIMINARY XRK7988 INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER
ABSOLUTE MAXIMUM RATINGSa
SYMBOL VCC VIN VOUT IIN IOUT TS Supply Voltage DC Input Voltage DC Output Voltage DC Input Current DC Output Current Storage Temperature -65
CHARACTERISTICS
MIN -0.3 -0.3 -0.3
MAX 3.6 VCC+0.3 VCC+0.3 +20 +50 125
UNIT V V V mA mA C
CONDITION
a. Absolute maximum continuous ratings are those maximum values beyond which damage to the device may occur. Exposure to these conditions or conditions beyond those indicated may adversely affect device reliability. Functional operation at absolute-maximum-rated conditions is not implied.
GENERAL SPECIFICATIONS
SYMBOL VTT MM HBM LU CIN JA CHARACTERISTICS Output termination voltage ESD Protection (Machine model) ESD Protection (Human body model) Latch-up immunity Input Capacitance Thermal resistance junction to ambient JESD 51-3, single layer test board JESD 51-6, 2S2P multilayer test board JC Thermal resistance junction to case Operating junction temperature 200 2000 200 4.0 MIN TYP VCC-2 MAX UNIT V V V mA pF Inputs CONDITION
62.0 47 14 115
C/W C/W C/W C
Natural convection Natural convection
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XRK7988 PRELIMINARY INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER DC CHARACTERISTICS (VCC= 3.3 + 5%, TA= -40C TO +85C)
SYMBOL CHARACTERISTICS MIN TYP MAX UNIT
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CONDITION
LVCMOS control inputs (MR, PLL_En, Sel_CLK, Man_Override, Alarm_Reset) VIH VIL IIN Input voltage high Input voltage low Input currenta 100 2.0 VCC+0.3 0.8 -150 V V VIN =VCC or VIN =GND
LVCMOS Control Outputs VOH VOL Output High Voltage Output Low Voltage 2.0 0.55 V V IOH=-10mA IOL=10mA
LVPECL clock inputs (CLK, CLK)b IIN Input current +100 VIN =VCC or VIN =GND
LVPECL clock outputs (Qa[1:0], Qa[1:0], Qb[2:0], Qb[2:0]) VOH VOL Output high voltage Output low voltage VCC-1.2 VCC-1.9 VCC-0.7 VCC-1.45 V V Termination 50 to VTT Termination 50 to VTT
Supply Current IGND ICCPLL Maximum ground supply current - gnd pins Maximum PLL power supply - VCC_PLL pin 180 15 mA mA GND pins VCCPLL pin
a. Inputs have internal pullup/pulldown resistors which affect the input current. b. Clock inputs driven by LVPECL compatible signals.
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PRELIMINARY XRK7988 INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER
AC CHARACTERISTICS (VCC= 3.3 + 5%, TA= -40C TO +85C) (NOTE 5)
SYMBOL fref fVCO fMAX PARAMETER Input Reference Frequency PLL VCO Lock Range Output Frequency Qa[1:0] Qa[1:0] frefDC tpd Reference Input Duty Cycle Propagation Delay CLKn to Ext_FB (SPO) c CLKn to Q (Bypass) VPP VCMR tskew Differential input voltage (peak-to-peak) Differential input crosspoint voltage Output Skew Within Qa[1:0] or Qb[2:0] All outputs Rate of change of periods Qa[1:0]d Qb[2:0]d Qa[1:0]e Qb[2:0]e tpw tjitter tlock tr/tf Output duty cycle Cycle-to-cyle jitter, Standard deviation (RMS) Maximum PLL lock time Output Rise/Fall time 50 45 50 25 400 200 55 40 10 700 % ps ms ps @fref = 18.75MHz ps/cycle -150 150 5 1.3 VCC-0.3 50 80 ps ns V V PLL_EN = 1 PLL_EN = 0 12.5 100 25 25.0 200 75 % /16 feedback MIN 12.5 200 TYP MAX 25.0 400 UNIT MHz MHz MHz CONDITION Locked Qa Output used for feedback
0.25 VCC-1.7
ps ps
per/cycle
c. Static phase offset between the selected reference clock and the feedback signal. d. Specification holds for a clock switch between two signals no greater than 400ps out of phase. Delta period change per cycle is averaged over the clock switch excursion. (See Applications Information section for more detail) e. Specification holds for a clock switch between two signals no greater than out of phase. Delta period change per cycle is averaged over the clock switch excursion. f. PECL output termination is 50 ohms to VCC - 2.0V. g. VPP is the minimum differential input voltage swing required to maintain AC characteristic including SPO, device and part-to-part skew. Applicable to CLK0, CLK1 and Ext_FB. h. VCMR is the crosspoint of the differential input signal. Normal operation is obtained when the crosspoint is within the VCMR range and the input swing lies within the VPP specification. Violation of VCMR or VPP impacts the SPO, device and part-to-part skew. Applicable to CLK0, CLK1 and Ext_FB.
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XRK7988 PRELIMINARY INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER APPLICATIONS INFORMATION The XRK7988 is a dual clock PLL with on-chip Intelligent Dynamic Clock Switch circuitry. DEFINITIONS primary clock: The input CLK selected by Sel_Clk. secondary clock: The input CLK NOT selected by Sel_Clk.
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PLL reference signal: The CLK selected as the PLL reference signal by Sel_Clk or the Intelligent Dynamic Clock Switch. The Intelligent Dynamic Clock Switch can override Sel_Clk. STATUS FUNCTIONS Clk_Selected: Clk_Selected (L) indicates CLK0 is selected as the PLL reference signal. Clk_Selected (H) indicates CLK1 is selected as the PLL reference signal. Inp0bad, Inp1bad: Inp0bad is latched (H) when CLK0 is stuck (H) or (L) for at least one Ext_FB period, or if one of the inputs CLK0 or CLK0 is floating. Inp1bad is latched (H) when CLK1 is stuck (H) or (L) for at least one Ext_FB period, or if one of the inputs CLK1 or CLK1 is floating. Both Inp0bad and Inp1bad are latched (H) when Ext_FB is stuck (H) or (L) for at least one Qa period, or if one of the inputs Ext_FB or Ext _FB is floating. Both Inp0bad and Inp1bad are cleared (L) on assertion of Alarm_Reset. The status functions Inp0bad and Inp1bad are active for Man_Override (H) or (L). CONTROL FUNCTIONS Sel_Clk: Sel_Clk (L) selects CLK0 as the primary clock. Sel_Clk (H) selects CLK1 as the primary clock. Alarm_Reset: Asserted by a negative edge. Generates a one-shot reset pulse that clears INPUT_BAD latches and Clk_Selected latch. PLL_En: While (L), the PLL reference signal is substituted for the VCO output. MR: While (L), internal dividers are held in reset which holds all Q outputs LOW. MAN OVERRIDE (H) (IDCS is disabled, PLL functions normally). PLL reference signal (as indicated by Clk_Selected) will always be the CLK selected by Sel_Clk. If Ext_FB misses at least one pulse, Qa and Qb outputs will drop to a minimum frequency (~20MHz) for 1-uS, or until Ext_FB shows any activity, whichever is longer. This prevents the Qa and Qb frequencies from rising due the PLL incorrectly interpreting an intermittent Ext_FB as a VCO running too slow. MAN OVERRIDE (L) Intelligent Dynamic Clock Switch is enabled. The first CLK to fail will latch it's INP_BAD (H) status flag and select the other input as the Clk_Selected for the PLL reference clock. Once latched, the Clk_Selected and INP_BAD remain latched until assertion of Alarm_Reset which clears all latches (INP_BADs are cleared and Clk_Selected = Sel_Clk). If both Inp0bad and Inp1bad are (H), either due to both CLK0 and CLK1 having missed at least 1 pulse each or Ext_FB having missed at least 1 pulse, then Qa and Qb outputs will drop to a minimum frequency (~20MHz) until such time as Alarm_Reset_b is asserted.
NOTE: If both CLKs are bad when Alarm_Reset is asserted, both INP_BADs will be latched (H) after one Ext_FB period and Clk_Selected will be latched (L) indicating CLK0 is the PLL reference signal. While neither INP_BAD is latched (H), the Clk_Selected can be freely changed with Sel_Clk. Whenever a CLK switch occurs, (manually or by the Intelligent Dynamic Clock Switch), following the next negative edge of the newly selected PLL reference signal, the next positive edge pair of Ext_FB and the newly selected PLL reference signal will slew to alignment.
To calculate the overall uncertainty between the input CLKs and the outputs from multiple XRK7988's, the following procedure should be used. Assuming that the input CLKs to all XRK7988's are exactly in phase, the total uncertainty will be the sum of the static phase offset, max I/O jitter, and output to output skew. During a dynamic switch, the output phase between two devices may be increased for a short period of time. If the two input CLKs are 400ps out of phase, a dynamic switch of an XRK7988 will result in an instantaneous phase change of 400ps to the PLL reference signal without a corresponding change in the output phase (due to the limited response of the PLL). As a result, the I/O phase of a device, undergoing this switch, will initially
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REV. P1.0.1
PRELIMINARY XRK7988 INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER
be 400ps and diminish as the PLL slews to its new phase alignment. This transient timing issue should be considered when analyzing the overall skew budget of a system. HOT INSERTION AND WITHDRAWAL In PECL applications, a powered up driver will experience a low impedance path through an XRK7988 input to its powered down VCC pins. In this case, a 100 ohm series resistance should be used in front of the input pins to limit the driver current. The resistor will have minimal impact on the rise and fall times of the input signals. ACQUIRING FREQUENCY LOCK 1. While the XRK7988 is receiving a valid CLK signal, assert Man_Override HIGH. 2. The PLL will phase and frequency lock within the specified lock time. 3. Apply a HIGH to LOW transition to Alarm_Reset to reset Input Bad flags. 4. De-assert Man_Override LOW to enable Intelligent Dynamic Clock Switch mode.
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XRK7988 PRELIMINARY INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER
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PACKAGE DIMENSIONS
32 LEAD THIN QUAD FLAT PACK (7 x 7 x 1.4 mm TQFP) rev. 2.00
D D1 24 17
25
16
D1 32 9
D
1 B A2 e
8
A Seating Plane A1 L
C
Note: The control dimension is the millimeter column INCHES SYMBOL A A1 A2 B C D D1 e L MIN 0.055 0.002 0.053 0.012 0.004 0.346 0.272 MAX 0.063 0.006 0.057 0.018 0.008 0.362 0.280 MILLIMETERS MIN 1.40 0.05 1.35 0.30 0.09 8.80 6.90 MAX 1.60 0.15 1.45 0.45 0.20 9.20 7.10
0.0315 BSC 0.018 0 0.030 7
0.80 BSC 0.45 0 0.75 7
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XRK7988 PRELIMINARY INTELLIGENT DYNAMIC CLOCK SWITCH PLL CLOCK DRIVER REVISION HISTORY
REVISION # A1.0.0 P1.0.0 P1.0.1 DATE August 2004 January 2005 March 2005 Initial release. DESCRIPTION
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REV. P1.0.1
Revised Electrical characteristics, Application Section, changed to Preliminary. Minor style edits.
NOTICE EXAR Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained here in are only for illustration purposes and may vary depending upon a user's specific application. While the information in this publication has been carefully checked; no responsibility, however, is assumed for inaccuracies. EXAR Corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless EXAR Corporation receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; (c) potential liability of EXAR Corporation is adequately protected under the circumstances. Copyright 2005 EXAR Corporation Datasheet March 2005. Reproduction, in part or whole, without the prior written consent of EXAR Corporation is prohibited.
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