74ABT5074 データシートの表示（PDF） - Philips Electronics

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74ABT5074

Synchronizing dual D-type flip-flop with metastable immune characteristics

Philips Electronics 

Philips Semiconductors

Synchronizing dual D-type flip-flop

with metastable immune characteristics

Product data

74ABT5074

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.

SIGNAL

GENERATOR

SIGNAL

GENERATOR

D

Q

CP Q

TRIGGER

DIGITAL

SCOPE

INPUT

After determining the T0 and τ of the flop, calculating the mean time

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:

MTBF = e(t’/τ)/ TO*fC*fI

In this formula, fC is the frequency of the clock, fI is the average

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

this situation the fI will be twice the data frequency of 20 MHz

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

Multiplying fI by fC gives an answer of 1015 Hz2. From Figure 2 it is

clear that the MTBF is greater than 1010 seconds. Using the above

formula the actual MTBF is 1.69 × 1010 seconds or about 535 years.

Figure 1. Test Setup

SA00004

E13

E6

E8

E10

E12 E14 E15 = fc*fi

E12

10,000 YEARS E11

MTBF

(SECONDS)

E10

100 YEARS E9

E8

ONE YEAR E7

E6

ONE WEEK E5

4

5

6

7

t’ (NANOSECONDS)

VCC = 5 V, Tamb = 25 °C, τ =94 ps, To = 1.3x107 sec MTBF = e(t’/τ)/TO*fC*fI

Figure 2. Mean Time Between Failures (MTBF) versus t’

8

SA00005

2002 Dec 17

4

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