Philips Semiconductors

Product data

74ABT5074

Synchronizing dual D-type flip-flop

with metastable immune characteristics

2002 Dec 17

4

METASTABLE IMMUNE CHARACTERISTICS

Philips Semiconductors uses the term ‘metastable immune’ to

describe characteristics of some of the products in its family. By

running two independent signal generators (see Figure 1) at nearly

the same frequency (in this case 10 MHz clock and 10.02 MHz data)

the device-under-test can often be driven into a metastable state. If

the Q output is then used to trigger a digital scope set to infinite

persistence the Q output will build a waveform. An experiment was

run by continuously operating the devices in the region where

metastability will occur.

DQ

Q

CP

TRIGGER

DIGITAL

SCOPE

INPUT

SIGNAL

GENERATOR

SA00004

SIGNAL

GENERATOR

Figure 1.

Test Setup

between failures (MTBF) is simple. Suppose a designer wants to

use the 74ABT5074 for synchronizing asynchronous data that is

arriving at 10 MHz (as measured by a frequency counter), has a

clock frequency of 50 MHz, and has decided that he would like to

sample the output of the 74ABT5074 7 nanoseconds after the clock

edge. He simply plugs his number into the following equation:

input event frequency, and t’ is the time after the clock pulse that the

output is sampled (t’ > h, h being the normal propagation delay). In

because input events consist of both of low and high transitions.

formula the actual MTBF is 1.69

E6

E8

E10

E12

E14

E15 = fc*fi

E13

E12

E11

E10

E9

E8

E7

E6

E5

10,000 YEARS

100 YEARS

ONE YEAR

ONE WEEK

MTBF

(SECONDS)

t’ (NANOSECONDS)

456

78

SA00005

Figure 2.

Mean Time Between Failures (MTBF) versus t’