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ID: 3740
Case Type: faq
Category: Inquiries
Related To: Datasheet
Family: ispMACH 4000

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What is the difference between "External feedback" and "internal feedback" when calculating Fmax for ispMACH 4000 devices?

When calculating the Fmax , the following two cases are considered
1. Internal feedback
2. External feedback

the signal is fed back through the GRP (global routing pool) without
going through an IO pad, it is internal feedback whereas when it is fed
back through an IO, it is external feedback.

Refer to the ispMACH 4000 Timing Model Design and Usage Guidelines Appnote for more information.

The example 2 on page 5 shows the calculation for Fmax when using internal feedback.
The Appnote is available under the path : > Products > FPGA & CPLD > ispMACH 4000ZE > Application Note 
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