The CY7B991 and CY7B992 Programmable Skew Clock Buffers
(PSCB) offer user selectable control over system clock functions.
These multiple output clock drivers provide the system integrator
with functions necessary to optimize the timing of high perfor-
mance computer systems. Each of the eight individual drivers,
arranged in four pairs of user controllable outputs, can drive
terminated transmission lines with impedances as low as 50Ω.
They can deliver minimal and specified output skews and full
swing logic levels (CY7B991 TTL or CY7B992 CMOS).
Each output is hardwired to one of the nine delay or function
configurations. Delay increments of 0.7 to 1.5 ns are determined
by the operating frequency with outputs that skew up to ±6 time
units from their nominal “zero” skew position. The completely
integrated PLL allows cancellation of external load and trans-
mission line delay effects. When this “zero delay” capability of the
PSCB is combined with the selectable output skew functions,
you can create output-to-output delays of up to ±12 time units.
Divide-by-two and divide-by-four output functions are provided
for additional flexibility in designing complex clock systems.
When combined with the internal PLL, these divide functions
enable distribution of a low frequency clock that are multiplied by
two or four at the clock destination. This facility minimizes clock
distribution difficulty, allowing maximum system clock speed and
flexibility.
All output pair skew <100 ps typical (250 maximum)
3.75 to 80 MHz output operation
User selectable output functions
❐
Selectable skew to 18 ns
❐
Inverted and non-inverted
❐
Operation at 1⁄2 and 1⁄4 input frequency
❐
Operation at 2x and 4x input frequency (input as low as 3.75
MHz)
Zero input to output delay
50% duty cycle outputs
Outputs drive 50Ω terminated lines
Low operating current
32-pin PLCC/LCC package
Jitter < 200 ps peak-to-peak (< 25 ps RMS)
■
■
■
■
■
■
Logic Block Diagram
TEST
FB
REF
FS
4F0
4F1
4Q0
SELECT
INPUTS
(THREE
LEVEL)
4Q1
SKEW
3Q0
3Q1
2Q0
MATRIX
2Q1
1Q0
1Q1
PHASE
FREQ
DET
VCO AND
TIME UNIT
GENERATOR
FILTER
3F0
3F1
SELECT
2F0
2F1
1F0
1F1
Cypress Semiconductor Corporation
Document Number: 38-07138 Rev. *B
•
198 Champion Court
•
San Jose
,
CA 95134-1709
•
408-943-2600
Revised June 22, 2007
CY7B991
CY7B992
Pin Configuration
PLCC/LCC
TEST
V
CCQ
GND
REF
3F0
2F1
FS
3
3F1
4F0
4F1
V
CCQ
V
CCN
4Q1
4Q0
GND
GND
5
6
7
8
9
10
11
12
4
2
1
32 31 30
29
28
27
2F0
GND
1F1
1F0
V
CCN
1Q0
1Q1
GND
GND
CY7B991
CY7B992
26
25
24
23
22
13
21
14 15 16 17 18 19 20
3Q1
3Q0
FB
V
CCN
V
CCN
2Q0
2Q1
Pin Definitions
Signal Name
REF
FB
FS
1F0, 1F1
2F0, 2F1
3F0, 3F1
4F0, 4F1
TEST
1Q0, 1Q1
2Q0, 2Q1
3Q0, 3Q1
4Q0, 4Q1
V
CCN
V
CCQ
GND
IO
I
I
I
I
I
I
I
I
O
O
O
O
PWR
PWR
PWR
Description
Reference frequency input. This input supplies the frequency and timing against which all functional
variations are measured.
PLL feedback input (typically connected to one of the eight outputs).
Three level frequency range select. See
Table 1.
Three level function select inputs for output pair 1 (1Q0, 1Q1). See
Table 2.
Three level function select inputs for output pair 2 (2Q0, 2Q1). See
Table 2.
Three level function select inputs for output pair 3 (3Q0, 3Q1). See
Table 2.
Three level function select inputs for output pair 4 (4Q0, 4Q1). See
Table 2.
Three level select. See
“Test Mode”
on page 4 under the
“Block Diagram Description”
on page 3.
Output pair 1. See
Table 2.
Output pair 2. See
Table 2.
Output pair 3. See
Table 2.
Output pair 4. See
Table 2.
Power supply for output drivers.
Power supply for internal circuitry.
Ground.
Document Number: 38-07138 Rev. *B
Page 2 of 19
CY7B991
CY7B992
Block Diagram Description
Phase Frequency Detector and Filter
The Phase Frequency Detector and Filter blocks accept inputs
from the reference frequency (REF) input and the feedback (FB)
input and generate correction information to control the
frequency of the Voltage Controlled Oscillator (VCO). These
blocks, along with the VCO, form a Phase Locked Loop (PLL)
that tracks the incoming REF signal.
Skew Select Matrix
The skew select matrix contains four independent sections. Each
section has two low skew, high fanout drivers (xQ0, xQ1), and
two corresponding three level function select (xF0, xF1) inputs.
Table 2
shows the nine possible output functions for each section
as determined by the function select inputs. All times are
measured with respect to the REF input assuming that the output
connected to the FB input has 0t
U
selected.
Table 2. Programmable Skew Configurations
[1]
Function Selects
1F1, 2F1,
3F1, 4F1
LOW
LOW
LOW
MID
MID
MID
HIGH
HIGH
HIGH
1F0, 2F0,
3F0, 4F0
LOW
MID
HIGH
LOW
MID
HIGH
LOW
MID
HIGH
Output Functions
1Q0, 1Q1,
2Q0, 2Q1
–4t
U
–3t
U
–2t
U
–1t
U
0t
U
+1t
U
+2t
U
+3t
U
+4t
U
3Q0, 3Q1
4Q0, 4Q1
VCO and Time Unit Generator
The VCO accepts analog control inputs from the PLL filter block.
It generates a frequency used by the time unit generator to
create discrete time units that are selected in the skew select
matrix. The operational range of the VCO is determined by the
FS control pin. The time unit (t
U
) is determined by the operating
frequency of the device and the level of the FS pin as shown in
Table 1.
Table 1. Frequency Range Select and t
U
Calculation
[1]
f
NOM
(MHz)
FS
[2, 3]
LOW
MID
HIGH
Min
15
25
40
Max
30
50
80
1
t
U
=
-----------------------
-
f
NOM
×
N
Divide by 2 Divide by 2
–6t
U
–4t
U
–2t
U
0t
U
+2t
U
+4t
U
+6t
U
Divide by 4
–6t
U
–4t
U
–2t
U
0t
U
+2t
U
+4t
U
+6t
U
Inverted
where N =
44
26
16
Approximate
Frequency (MHz) At
Which t
U
= 1.0 ns
22.7
38.5
62.5
Notes
1. For all tri-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 is set on FS 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 1Q0 and the other outputs when they are operated in their undivided modes (see
Table 2).
The frequency appearing at the REF and FB
inputs are fNOM when the output connected to FB is undivided. The frequency of the REF and FB inputs are fNOM/2 or fNOM/4 when the part is configured for a
frequency multiplication by using a divided output as the FB input.
3. When the FS pin is selected HIGH, the REF input must not transition upon power up until VCC has reached 4.3V.
Document Number: 38-07138 Rev. *B
Page 3 of 19
CY7B991
CY7B992
Figure 1
shows the typical outputs with FB connected to a zero skew output.
[4]
Figure 1. Typical Outputs with FB Connected to a Zero-Skew Output
t
0
– 6t
U
t
0
– 5t
U
t
0
– 4t
U
t
0
– 3t
U
t
0
– 2t
U
t
0
– 1t
U
U
U
U
U
U
U
t
0
+1t
t
0
+2t
t
0
+3t
t
0
+4t
t
0
+5t
FBInput
REFInput
1Fx
2Fx
(N/A)
LL
LM
LH
ML
MM
MH
HL
HM
HH
(N/A)
(N/A)
(N/A)
3Fx
4Fx
LM
LH
(N/A)
ML
(N/A)
MM
(N/A)
MH
(N/A)
HL
HM
LL/HH
HH
– 6t
U
– 4t
U
– 3t
U
– 2t
U
– 1t
U
0t
U
+1t
U
+2t
U
+3t
U
+4t
U
+6t
U
DIVIDED
INVERT
Test Mode
The TEST input is a three level input. In normal system
operation, this pin is connected to ground, enabling the
CY7B991 or CY7B992 to operate as explained in
“Skew Select
Matrix”
on page 3. For testing purposes, any of the three level
inputs can have a removable jumper to ground, or be tied LOW
through a 100Ω resistor. This enables an external tester to
change the state of these pins.
If the TEST input is forced to its MID or HIGH state, the device
operates with its internal phase locked loop disconnected, and
input levels supplied to REF directly controls all outputs. Relative
output to output functions are the same as in normal mode.
In contrast with normal operation (TEST tied LOW), all outputs
function based only on the connection of their own function
selects inputs (xF0 and xF1) and the waveform characteristics of
the REF input.
Note
4. FB connected to an output selected for “zero” skew (i.e., xF1 = xF0 = MID).
Document Number: 38-07138 Rev. *B
t
0
+6t
t
0
Page 4 of 19
CY7B991
CY7B992
Maximum Ratings
Operating outside these boundaries affects the performance and
life of the device. These user guidelines are not tested.
Storage Temperature ................................. –65
°
C to +150
°
C
Ambient Temperature with
Power Applied ............................................ –55
°
C to +125
°
C
Supply Voltage to Ground Potential................–0.5V to +7.0V
DC Input Voltage ............................................–0.5V to +7.0V
Output Current into Outputs (LOW) ............................. 64 mA
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