Philips N74F50728N, N74F50728D Datasheet

INTEGRATED CIRCUITS

74F50728

Synchronizing cascaded dual positive edge-triggered D-type flip-flop

Positive specification IC15 Data Handbook

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1990 Sep 14

Philips Semiconductors Product specification

Synchronizing cascaded dual positive edge-triggered D-type flip-flop

FEA TURES

•Metastable immune characteristics

•Output skew less than 1.5ns

•See 74F5074 for synchronizing dual D-type flip-flop

•See 74F50109 for synchronizing dual J–K positive edge-triggered

flip-flop

•See 74F50729 for synchronizing dual dual D-type flip-flop with

edge-triggered set and reset

•Industrial temperature range available (–40°C to +85°C)

DESCRIPTION

The 74F50728 is a cascaded dual positive edge–triggered D–type featuring individual data, clock, set and reset inputs; also true and complementary outputs.

Set (S

Dn) and reset (RDn) are asynchronous active low inputs and operate independently of the clock (CPn) input. They set and reset both flip–flops of a cascaded pair simultaneously. Data must be stable just one setup time prior to the low–to–high transition of the clock for guaranteed propagation delays.

74F50728

Clock triggering occurs at a voltage level and is not directly related to the transition time of the positive–going pulse. Following the hold time interval, data at the Dn input may be changed without affecting the levels of the output. Data entering the 74F50728 requires two clock cycles to arrive at the outputs.

The 74F50728 is designed so that the outputs can never display a metastable state due to setup and hold time violations. If setup time and hold time are violated the propagation delays may be extended beyond the specifications but the outputs will not glitch or display a metastable state. Typical metastability parameters for the 74F50728 are: τ ≅ 135ps and T function of the rate at which a latch in a metastable state resolves that condition and T the propensity of a latch to enter a metastable state.

TYPE

74F50728 145 MHz 23mA

≅ 9.8 X 10

represents a function of the measurement of

TYPICAL f

sec where τ represents a

TYPICAL SUPPL Y

max

CURRENT (TOTAL)

ORDERING INFORMATION

ORDER CODE

COMMERCIAL RANGE INDUSTRIAL RANGE

DESCRIPTION

14–pin plastic DIP N74F50728N I74F50728N SOT27-1

14–pin plastic SO N74F50728D I74F50728D SOT108-1

VCC = 5V ±10%, VCC = 5V ±10%,

= 0°C to +70°C T

amb

= –40°C to +85°C

amb

PKG DWG #

INPUT AND OUTPUT LOADING AND FAN OUT TABLE

PINS DESCRIPTION

D0, D1 Data inputs 1.0/0.417 20µA/250µA CP0, CP1 Clock inputs (active rising edge) 1.0/1.0 20µA/20µA SD0, SD1 Set inputs (active low) 1.0/1.0 20µA/20µA RD0, RD1 Reset inputs (active low) 1.0/1.0 20µA/20µA

Q0, Q1, Q0, Q1 Data outputs 50/33 1.0mA/20mA

NOTE: One (1.0) FAST unit load is defined as: 20µA in the high state and 0.6mA in the low state.

74F (U.L.) HIGH/

LOW

LOAD VALUE HIGH/

LOW

September 14, 1990 853-1389 00421

Philips Semiconductors Product specification

Synchronizing cascaded dual positive edge-triggered D-type flip-flop

PIN CONFIGURATION

SF00605

212

D1D0

R D1

12 11

CP1

SD1

SF00606

D0 D0

CP0

Q0 Q

GND

LOGIC SYMBOL

3 4

1 11 10

VCC = Pin 14 GND = Pin 7

IEC/IEEE SYMBOL

1 2 3 4 5 6

CP0 SD0

RD0 CP1 SD1

RD1

Q0 Q0 Q1 Q1

56 98

74F50728

LOGIC DIAGRAM

4, 10

SDn

CPn

Vcc = Pin 14 GND = Pin 7

2, 12

3, 11

1, 13

NOTE: Data entering the flip–flop requires two clock cycles to

arrive at the output.

SYNCHRONIZING SOLUTIONS

Synchronizing incoming signals to a system clock has proven to be costly, either in terms of time delays or hardware. The reason for this is that in order to synchronize the signals a flip–flop must be used to ”capture” the incoming signal. While this is perhaps the only way to synchronize a signal, to this point, there have been problems with this method. Whenever the flop’s setup or hold times are violated the flop can enter a metastable state causing the outputs in turn to glitch, oscillate, enter an intermediate state or change state in some abnormal fashion. Any of these conditions could be responsible for causing a system crash. T o minimize this risk, flip–flops are often cascaded so that the input signal is captured on the first clock pulse and released on the second clock pulse (see Fig.1). This gives the first flop about one clock period minus the flop delay and minus the second flop’s clock–to–Q setup time to resolve any metastable condition. This method greatly reduces the probability of the outputs of the synchronizing device displaying an abnormal state but the trade-off is that one clock cycle is lost to synchronize the incoming data and two separate flip–flops are required to produce the cascaded flop circuit. In order to assist the designer of synchronizing circuits Philips Semiconductors is offering the 74F50728.

DATA

CLOCK

D Q

SF00608

Q OUTPUT

5, 9

6, 8

Q n

September 14, 1990

SF00607

SF00609

Figure 1.

The 50728 consists of two pair of cascaded D–type flip–flops with metastable immune features and is pin compatible with the 74F74. Because the flops are cascaded on a single part the metastability

Philips Semiconductors Product specification

Synchronizing cascaded dual positive edge-triggered D-type flip-flop

characteristics are greatly improved over using two separate flops that are cascaded. The pin compatibility with the 74F74 allows for plug–in retrofitting of previously designed systems.

Because the probability of failure of the 74F50728 is so remote, the metastability characteristics of the part were empirically determined based on the characteristics of its sister part, the 74F5074. The table below shows the 74F5074 metastability characteristics.

Having determined the T

and τ of the flop, calculating the mean

time between failures (MTBF) for the 74F50728 is simple. It is, however, somewhat dif ferent than calculating MTBF for a typical part because data requires two clock pulses to transit from the input to the output. Also, in this case a failure is considered of the output beyond the normal propagation delay.

TYPICAL VALUES FOR τ AND T0 AT VARIOUS VCCS AND TEMPERA TURES

= 0°C

amb

τ T VCC = 5.5V 125ps 1.0 X 109 sec 138ps 5.4 X 106 sec 160ps 1.7 X 105 sec VCC = 5.0V 115ps 1.3 X 1010 sec 135ps 9.8 X 106 sec 167ps 3.9 X 104 sec VCC = 4.5V 115ps 3.4 X 1013 sec 132ps 5.1 X 108 sec 175ps 7.3 X 104 sec

Suppose a designer wants to use the flop for synchronizing asynchronous data that is arriving at 10MHz (as measured by a frequency counter), and is using a clock frequency of 50MHz. He simply plugs his number into the equation below:

MTBF = e

(t’/t)

/TofCf

In this formula, fC is the frequency of the clock, fI is the average input event frequency , and t’ is the period of the clock input (20 nanoseconds). In this situation the f

will be twice the data

frequency of 20 MHz because input events consist of both of low and high data transitions. From Fig. 2 it is clear that the MTBF is greater than 10 MTBF is 2.23 X 10

amb

τ T

= 25°C

seconds. Using the above formula the actual

seconds or about 7 X 1034 years.

amb

τ T

74F50728

= 70°C

MEAN TIME BETWEEN FAILURES VERSUS DATA FREQUENCY AT VARIOUS CLOCK FREQUENCY

Clock = 40MHz

Clock = 50MHz

Clock = 650MHz

Clock = 70MHz

Clock = 80MHz

Clock = 100MHz

Data frequency (Hz)

SF00610

Figure 2.

NOTE: V

= 5V, T

Mean time

between failures

(seconds)

1 billion years

= 25°C, τ =135ps, To = 9.8 X 108 sec

amb

1K 100K 10M

September 14, 1990

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