Metastability Characterization Report for Actel Flash FPGAs
v1.0
22- 5
The metastability theory indicates that C1 and C2 are independent of the test clock and data
frequency. The test results concur within experimental tolerances. The calculations of C1 and C2 are
Examples of Metastability Coefficients Usage
Metastability shows a statistical nature, and designers should allow enough additional time (Tmet)
that the likelihood of metastable failure is remote enough to be tolerable by the design
specification.
ProASICPLUS device to synchronize an asynchronous data input to the FPGA. The following
parameters are given to designer by either design specification or post-layout timing analysis:
Tco = 10 ns, corresponding to a clock frequency of 100 MHz
Asynchronous data transition rate = 12.5 MHz
Tolerable MTBF = 1 year
If the designer does not allow additional sampling time (Tmet = 0 ns) and run the clock at the rate
error will occur at the output of the second flip-flop every 51.2 s. This value exceeds the required
MTBF of one year indicated in the design specification. To meet this requirement, the designer
needs to allow additional Tmet in the sampling time, which can be calculated as follows:
1 year = 365 × 24 × 3,600 = 31,536,000 seconds
ln 31,536,000 = 9.148E+09 × Tmet – ln (1.56E–11 × 100E6 × 12.5E6) ≥ Tmet = 2.96 ns
Therefore, an additional 3 ns sampling time will fulfill the required MTBF.
Part Number and Revision Date
This document was previously published as an application note describing features and functions
of the device, and as such has now been incorporated into the device handbook format. No
technical changes have been made to the content.
Part Number 51700094-023-0
Revised January 2008
List of Changes
The following table lists critical changes that were made in the current version of the chapter.
Table 22-1 Metastability Coefficients for Actel Flash FPGAs
fc = 10 MHz
Device Family
C1 (s)
C2 (s)
ProASIC
9.95E–11
1.03E+10
ProASICPLUS
1.56E–11
9.148E+09
ProASIC3/E Core Registers
9.11E–12
1.57E+10
ProASIC3/E I/O Registers
2.25E–12
1.91E+10
Previous Version
Changes in Current Version (v1.0)
Page
5190062-1/5.04
include ProASIC3/E information.
5190062-0
This document was updated to provide a detailed description of the
calculations being made.