AR# 51561
7 Series - Manual Timing Adjustment required when using a MMCM with Spread Spectrum Clocks.
Description
The 7 series MMCM adds spread spectrum clock generation using both center spread and down spread.
fig1.png
Figure 1: Center-Spread Modulation
fig2.png
Figure 2: Down-spread Modulation
When using spread spectrum in SS_MODE(CENTER_HIGH) or SS_MODE( CENTER_LOW), the center spread will increase the effective frequency ranges.
Solution
The user must manually constrain the input clock to account for the range of frequencies within the Spread Spectrum.
The table below gives the input frequency adjustment formula (FIN_SS) that is required based on the Spread Spectrum modes selected (SS_MODE), the Input Frequency (FIn) and the Multiply value selected (M).
Input Frequency [MHz] |
M |
Input Frequency Adjustment (FIN_SS) | |
SS_MODE(CENTER_HIGH) |
25 < FIN < 35 |
M=28 |
FIN_SS = FIN * 56 / 55 |
35 < FIN < 50 |
M=21 |
FIN_SS = FIN * 42 / 41 | |
M=22 |
FIN_SS = FIN * 44 / 43 | ||
50 < FIN < 75 |
M=28 |
FIN_SS = FIN * 56 / 55 | |
75 < FIN < 150 |
M=21 |
FIN_SS = FIN * 42 / 41 | |
M=22 |
FIN_SS = FIN * 44 / 43 | ||
SS_MODE (CENTER_LOW) |
25 < FIN < 35 |
M=56 |
FIN_SS = FIN * 112 / 111 |
35 < FIN < 50 |
M=42 |
FIN_SS = FIN * 84 / 83 | |
M=44 |
FIN_SS = FIN * 88 / 87 | ||
50 < FIN < 75 |
M=56 |
FIN_SS = FIN * 112 / 111 | |
75 < FIN < 150 |
M=42 |
FIN_SS = FIN * 84 / 83 | |
M=44 |
FIN_SS = FIN * 88 / 87 | ||
SS_MODE (DOWN_HIGH) |
25 < FIN < 35 |
M=28 |
FIN_SS = FIN |
35 < FIN < 50 |
M=21, 22 |
FIN_SS = FIN | |
50 < FIN < 75 |
M=28 |
FIN_SS = FIN | |
75 < FIN < 100 |
M=21, 22 |
FIN_SS = FIN | |
100 < FIN < 150 |
M=21, 22 |
FIN_SS = FIN | |
SS_MODE (DOWN_LOW) |
25 < FIN < 35 |
M=56 |
FIN_SS = FIN |
35 < FIN < 50 |
M=42, 44 |
FIN_SS = FIN | |
50 < FIN < 75 |
M=56 |
FIN_SS = FIN | |
75 < FIN < 100 |
M=42, 44 |
FIN_SS = FIN | |
100 < FIN < 150 |
M=42, 44 |
FIN_SS = FIN |
With a 25 MHz input clockthenew timing constraint would be as follows:
SS_MODE(CENTER_HIGH) = 25 * 56/55 = 25.45 MHz
SS_MODE (CENTER_LOW) = 25 * 112/111 = 25.23 MHz
SS_MODE (DOWN_HIGH) and SS_MODE (DOWN_LOW) as the Spread will reduce the frequency, then no change is needed to the input Clock.
With an 80 MHz clock with M = 22 for (CENTER_HIGH) and M = 44 for (CENTER_LOW):
SS_MODE(CENTER_HIGH) =80 *44/43 = 81.86 MHz
SS_MODE (CENTER_LOW) =80*88/87 = 80.92 MHz
Again, as for SS_MODE (DOWN_HIGH) and SS_MODE (DOWN_LOW), as the Spread will reduce the frequency, then no change is needed to the input Clock.
The above will be required for all versions of ISE Design Suite. Vivado Design Suite 2013.1onwards has support for adjusting the timing constraints automaticallybased on the Spread Spectrum clocking usage.
Linked Answer Records
Associated Answer Records
| Answer Number | Answer Title | Version Found | Version Resolved |
|---|---|---|---|
| 47351 | 7 Series MMCM Spread Spectrum support details | N/A | N/A |