P1P8160A Spread Spectrum Solution to AMD Graphic Chips with GDDR5 Memory Support

AND9017
ON Semiconductor
P1P8160A: Spread
Spectrum Clock Solution to
Address Peak EMI in AMD
Graphics Platforms with
GDDR Memory
http://onsemi.com
APPLICATION NOTE
Prepared by: Jay Hsu
ON Semiconductor
Abstract
Active Spread Spectrum (SS) technology modulates the
frequency to suppress peak EMI and also ensures signal
integrity is not degraded in the GDDR clock signal by
preserving the rise/fall times. All signals derived from this
SS clock will be modulated and peak EMI levels are
suppressed. SSCG technology is an optimal solution to solve
this peak EMI problem. P1P8160A is the ideal SSCG timing
solution for AMD’s Northern Islands and Evergreen series
graphic cards with GDDR3 and GDDR5 memory support.
The P1P8160A provides two clocks: a 27 MHz reference
clock to GPU core, and a 100 MHz SS clock to the GDDR
memory.
Graphics Processing Units (GPU) running with Graphics
Double Data Rate memories, Version 3 and 5
(GDDR3/GDDR5), could potentially cause peak
Electro−Magnetic interference (EMI) failure at compliance
certification. This article is intended to address and solve
this peak EMI failure using ON Semiconductor’s new
Spread
Spectrum
Clock
Generator
(SSCG)
device*P1P8160A, specifically designed for use with AMD
Northern Islands/ Evergreen series GPUs.
Introduction
GDDR3 and GDDR5 memories are commonly used in
high performance graphics cards and run at very high
frequencies. If memory capacity goes up to 1 GB GDDR3
and GDDR5, it requires eight memory modules with eight
pairs of differential memory clocks from the GPU to the
GDDR3 and GDDR5 memory modules. These clocks are a
major source of EMI problems. The EMI certification tests
come very late in the product cycle and can cause
unexpected delays if the system fails EMI compliance.
© Semiconductor Components Industries, LLC, 2011
July, 2011 − Rev. 1
Using P1P8160A for AMD Graphic Chips Platforms
P1P8160A uses a 27 MHz fundamental crystal or an
external reference clock to generate a 27 MHz (no SS) and
a 100 MHz (SS) clock output. As shown in Figure 1, the
100 MHz SS clock will supply the reference clock to the
internal GDDR controller.
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Figure 1. P1P8160A for AMD Graphic Chips Application
P1P8160A: Key Functional Characteristics
allows to user to customize the modulation depth such that
P1P8160A provides the maximum EMI benefit without
violating system timing specifications. The deviation
settings of the P1P8160A are shown in Table 1.
P1P8160A provides eight different user selectable SS
deviation options to control the extent of frequency
modulation by setting Pin 3 (SS1%) and Pin 7 (SS2%) to one
of three levels (High, Medium, or Low). This flexibility
Table 1. P1P8160A: FREQUENCY DEVIATION SELECTION
SS2% (Pin 3)
SS1% (Pin 7)
Deviation at 100 MHz (%) (Pin 5)
L
L
SSOFF
L
M
−0.5
L
H
−2.5
M
L
−0.25
M
M
−0.75
M
H
−1
H
L
−1.5
H
M
−2
H
H
−3
P1P8160A: AC Electrical Characteristics
The P1P8160A AC Electrical Characteristics for the
27 MHz and 100 MHz output clocks are shown in Table 2.
These clocks meet AMD’s GPU clock specifications.
Table 2. P1P8160A OUTPUT CLOCKS AC ELECTRICAL CHARACTERISTICS
Symbol
fOUT
Parameter
Min
Typ
ModOUT Clock frequency (SS1% & SS2% = 0) (Tolerance: ±30 ppm)
100
RefOUT Clock frequency (Tolerance: ±30 ppm)
27
Max
tLH, tHL
RefOUT Rise and Fall time, CL = 15 pF
(Measured between 20% to 80%)
1.25
2
tLH, tHL
ModOUT Rise and Fall time, CL = 15 pF
(Measured between 20% to 80%)
1.25
1.75
TDCOUT
Output Clock Duty Cycle
50
55
45
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Unit
MHz
ns
%
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Table 2. P1P8160A OUTPUT CLOCKS AC ELECTRICAL CHARACTERISTICS
Symbol
Typ
Max
TJC
Cycle−Cycle Jitter (For ModOUT, RefOUT)
125
200
TJL
Long Term Jitter (10k cycles), 27 MHz, RefOUT
150
300
Long Term Jitter (10k cycles), 100 MHz ModOUT (SSOFF)
350
600
32
33
MF
Parameter
Min
Modulation Frequency
31
100 MHz Clock Output EMI Comparison
Unit
ps
kHz
without SS. As seen in Table 3, the EMI reduction in the
30 MHz− 1.3 GHz frequency range is 2 − 10 dB better with
−0.5% down spreading, when compared to SS turned off.
Device settings are: VDD = 3.3 V, Deviation = −0.5% and
Modulation rate = 32 kHz. Application testing was done
using Anritsu MS2711D Spectrum Analyzer.
The plots in Figure 2 display the results of EMI bench
testing using the P1P8160A. This test is performed on a
characterization board and the EMI is measured on the
100 MHz output clock using a spectrum analyzer. The
results show the difference in peak EMI levels, with and
Figure 2. 100 MHz EMI Spectrum Analyzer performance
Table 3. EMI PERFORMANCE TABLE
Freq
EMI P1P8160A SSOFF (dBm)
EMI P1P8160A SSON = −0.5% (dBm)
EMI Reduction (dB)
100 MHz
−5.16
−7.46
2.3
300 MHz
−10.58
−15.73
5.15
500 MHz
−13.01
−19.51
6.5
700 MHz
−16.17
−23.33
7.16
900 MHz
−18.33
−26.58
8.25
1100 MHz
−23.48
−32.55
9.07
1300 MHz
−30.71
−41.63
10.92
Validation in Barts (HD6870) System
The P1P8160A meets the AMD GPU clock
specifications. Measurements were performed using a
Tektronix TDS6604B Digital Oscilloscope, P7260 Active
Probes and TDSJITV3 Jitter Analysis Software.
This case study is based on an AMD Radeon HD6870
(DDR5 GPU) with a P1P8160A. The 27 MHz output clock
AC electrical characteristics are shown in Table 4 and the
output waveforms are shown in Figures 3a, 3b and 3c.
Table 4. APPLICATION TEST FOR P1P8160A: 27 MHZ OUTPUT CLOCK
P1P8160A
Rise Time
Fall Time
1.25 ns
1.34 ns
Duty Cycle
48% (min)
Cycle−Cycle Jitter
51% (max)
−183 ps (min)
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139 ps (max)
Long Term Jitter
200 ps
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a. Rise/Fall Time, Duty Cycle
b. Cycle−Cycle Jitter
c. Long Term Jitter
Figure 3.
Table 5 and Figures 4a, 4b and 4c show the 100 MHz
output clock Rise/ Fall time, Duty Cycle, Cycle−Cycle jitter
and Long term jitter measured values and waveforms. Long
term jitter is with SS turned OFF and the other parameters
are measured with SS ON (−0.5% down spreading). The
P1P8160A meets the AMD GPU clock specifications.
Table 5. APPLICATION TEST FOR P1P8160A: 100 MHz OUTPUT CLOCK
P1P8160A
Rise Time
Fall Time
0.785 ns
0.838 ns
a. Rise/Fall Time, Duty Cycle
Duty Cycle
48% (min)
Cycle−Cycle Jitter
52% (max)
b. Cycle−Cycle Jitter
−97.4 ps
(min)
85.8 ps
(max)
Long Term Jitter
(SSOFF)
300 ps
c. Long Term Jitter
Figure 4.
lowered by 9.5 dB when −0.5% (down spreading) SS is used
at 1.225 GHz. Testing was performed using Anritsu
MS2711D Spectrum Analyzer.
The EMI Spectrum Analyzer performance comparison
with the original non−SS DDR5 clock and a DDR5 SS Clock
is shown in Figure 5. The results show that the EMI peak is
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Reduction 9.5 dB
Figure 5. DDR
Clock EMI Performance
2. On the 100MHz output clock, the measured
Cycle*Cycle Jitter values are *97.4 ps(min) to
85.8 ps (max) and long term jitter is 300 ps.
The peak EMI comparison on the GDDR5clock shows
that the EMI is reduced by 9.5 dB at 1.225 GHz when −0.5%
down spread is used. The P1P8160A device is an efficient
and cost effective way to reduce the EMI peaks in systems
that use GDDR3 and GDDR5 memories.
Summary
Based on this case study, the Peak EMI benefits that can
be expected with −0.5%(down spreading) SS when using the
P1P8160A are:
1. 2 -3 dB, In 30−200 MHz frequency range
2. 5-9 dB, In 200−900 MHz frequency range
3. 9-10dB, In 1100−1300 GHz frequency range
The P1P8160A was designed to meet AMD clock
specifications and this case study highlights the benefits of
using the P1P8160A in AMD GPUs.
The key jitter specs are as follows:
1. On the 27 MHz output clock, the measured
Cycle*Cycle Jitter values are −183 ps(min) to
139 ps(max) and long term jitter is 200 ps.
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