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.

No versions of ISE Design Suite support automatic timing analysis for CENTER_HIGH or CENTER_LOW. Current versions Vivado Design Suite also do not support automatic timing analysis for CENTER_HIGH or CENTER_LOW.

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
	35 < FIN < 50	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
	75 < FIN < 150	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
	35 < FIN < 50	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
	75 < FIN < 150	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.

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Associated Answer Records

Answer Number	Answer Title	Version Found	Version Resolved
47351	7 Series MMCM Spread Spectrum support details	N/A	N/A

AR# 51561
Date	02/12/2013
Status	Active
Type	General Article
Devices	Artix-7 Kintex-7 Virtex-7 Virtex-7 HT
Tools	ISE Design Suite - 14 ISE Design Suite - 13 Vivado - 2012.2

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