W132 Datasheet, PDF (3/6 Pages) Cypress Semiconductor – Spread Aware, Ten/Eleven Output Zero Delay Buffer

W132
VDD
VDD
0.1µF
0.1µF
1 AGND
2 VDD
3 Q0
4 Q1
5 Q2
6 GND
7 GND
8 Q3
9 Q4
10 VDD
11 OE
12 FBOUT
GND 24
AVDD 23
VDD 22
0.1µF
Q9 21
Q8 20
GND 19
GND 18
Q7 17
Q6 16
Q5 15
VDD 14
FBIN 13
10µF
FB
0.1µF
10µF
FB
VDD
3.3V
0.1µF
VDD
Figure 1. Schematic
Spread Aware
Many systems being designed now utilize a technology called
Spread Spectrum Frequency Timing Generation. Cypress has
been one of the pioneers of SSFTG development, and we de-
signed this product so as not to filter off the Spread Spectrum
feature of the Reference input, assuming it exists. When a
zero delay buffer is not designed to pass the SS feature
through, the result is a significant amount of tracking skew
which may cause problems in systems requiring synchroniza-
tion.
For more details on Spread Spectrum timing technology,
please see the Cypress application note titled, “EMI Suppres-
sion Techniques with Spread Spectrum Frequency Timing
Generator (SSFTG) ICs.”
How to Implement Zero Delay
Typically, zero delay buffers (ZDBs) are used because a de-
signer wants to provide multiple copies of a clock signal in
phase with each other. The whole concept behind ZDBs is that
the signals at the destination chips are all going HIGH at the
same time as the input to the ZDB. In order to achieve this,
layout must compensate for trace length between the ZDB and
the target devices. The method of compensation is described
below.
External feedback is the trait that allows for this compensation.
The PLL on the ZDB will cause the feedback signal to be in
phase with the reference signal. When laying out the board,
match the trace lengths between the output being used for
feed back and the FBIN input to the PLL.
If it is desirable to either add a little delay, or slightly precede
the input signal, this may also be affected by either making the
trace to the FBIN pin a little shorter or a little longer than the
traces to the devices being clocked.
Inserting Other Devices in Feedback Path
Another nice feature available due to the external feedback is
the ability to synchronize signals up to the signal coming from
some other device. This implementation can be applied to any
device (ASIC, multiple output clock buffer/driver, etc.) which is
put into the feedback path.
Referring to Figure 2, if the traces between the ASIC/buffer
and the destination of the clock signal(s) (A) are equal in length
to the trace between the buffer and the FBIN pin, the signals
at the destination(s) device will be driven high at the same time
the Reference clock provided to the ZDB goes high. Synchro-
nizing the other outputs of the ZDB to the outputs form the
ASIC/Buffer is more complex however, as any propagation de-
lay in the ASIC/Buffer must be accounted for.
Reference
Signal
Feedback
Input
Zero
Delay
Buffer
ASIC/
Buffer
A
Figure 2. 6 Output Buffer in the Feedback Path
Document #: 38-07216 Rev. *A
Page 3 of 6