Logic Family Propagation delay

Slew Rate Definition Graph
IC Propagation Delay, I/O

IC Propagation Delay

This page provides a short discussion of IC Propagation delay [Tpd] and how it varies as the environment or individual designs vary. Any numbers provided to indicate changes in Propagation delay are general and only used to show a relationship. Real Propagation delay numbers, or changes in Propagation delay will depend on the logic family or device used. There are dozens of different logic families, all with different propagation delays, input load values, and other characteristics. The point here is to show that if a circuit design requires the correct characteristic to operate correctly, then these additional considerations need to be accounted for. In general the propagation delay of an IC will differ from what is shown in the data sheet because of one or more of the changes in operational characteristics discussed below. However the data sheet for a device may account for some of these changes [operational temperature, Vcc variation], but usually not for increases in load capacitance.

Propagation delay increases with operating temperature. Some IC families change approximately 2nS over the entire temperature range of -50oC to +100oC. Some data sheets may only provide a min/max propagation delay at 25oC.

A related issue described on another page is Slew Rate, or the time it takes for an output to change from high to low.

Propagation delay may also be affected by the number of outputs switching simultaneously. The 74FCTxx family adds an additional 250pS to the Propagation delay for each output in a package switching [above the first one]. The data sheet may be providing min/max Propagation delay for one output switching or all possible outputs switching at once. If the device has 8 or 16 outputs the Propagation delay could change by a large percentage.

Power supply, Vcc variations may also effect the Propagation delay of a device. A change of 0.5nS may be expected as Vcc varies between it's minimum and maximum values. I believe it's normally inferred that this is accounted for over the normal allowable Vcc provided in the data sheet. Again, if the circuit design is operating at a speed at which Propagation delay needs to be accounted for ~ then these variations need to be accounted for.

The largest change in Propagation delay will be due to change in output Load, CL. Normally a typical value of CL is provided in the data sheet, the value for Propagation delay is based on this load. As the load is increased the Propagation delay increases. Some families which provide a Propagation delay number at 50pF [CL] show an increase of 1nS per increase in load [or for each additional 50pF load]. Or, a device may have a 5nS Propagation delay with a 50pF load, 6nS Propagation delay with a 100pF load, a 7nS Propagation delay with a 150pF load and so on. So the point here is the data sheet provides a valid Propagation delay for driving one load [1 device], if the output is driving more then one load then the Propagation delay is vary different [larger]. Additional increases in load capacitance may include trace capacitance or connector capacitance or what ever happens to be on that net..

One other factor which may increase Propagation delay could be induced by adding a series termination resistor to reduce reflections on the line.
Line terminations are discussed on the Trace Termination page.

So if your asking why do I care about IC propagation delay or does IC output propagation delay effect may design, or how do changes in propagation delay effect my design then you may need to account for the variations already discussed.
Intermittent operation, or failures may be seen if the variation discussed above have not been accounted for. In the lab, the power supply was always re-adjusted [for each test] to provide the correct Vcc regardless of load, while the fielded board [PWB] does not. In the lab the temperature is always 25oC, the fielded board resides in a Humvee out in the desert. My test code pushed 2 data bits out this IC, but the real data switches all 8 bits. There were two boards [PWBs] in the test chassis while 4 boards reside in the operational chassis. If the minimum and maximum values for propagation delay is not accounted for then one board may work [having one delay] while another board with the same design may not work, because the propagation delay changed with the different chip used. Propagation delay will change from IC to IC between the listed minimum and maximum values listed on the data sheet.

The issue is a problem but may not show as a failure for all fielded boards, or for boards fielded to a particular region, or it will just show as a random error with no correlation. So the rule is; follow the design rules for the particular logic family used or after the fact [once the design is done] determine what is causing the random errors [which is much harder]. Account for the propagation delay changing between the minimum and maximum values provided in the data sheet, then account for another other issue which may tend to increase the propagation delay. Don't rely on a typical value with out accounting for the variation between minimum and maximum values. Intermittent circuit operation may be seen when switching times are not accounted for.

System Propagation Delay

How to design determine propagation delay within systems or interconnecting circuits.

How To Determine Propagation Delays
Propagation Delay Example

Each component in the system adds propagation delay to the signal, not just the Integrated Circuit [IC].
The graphic above shows an example backplane interface, with slot 1 transmitting to slot 5 [blue transceivers].
The individual Printed Circuit Board [PCB] contains the transceiver IC, with a one inch trace [light blue] to the board connector, and then the physical connector itself [gray].
Each card is designed the same, with the ICs as close as possible to the backplane connector, 1 inch in this example.

The backplane connectors are spaced at a standard 0.8 inches between boards.
The 0.8 inch board-to-board spacing is common in a large number of backplane formats.
Because the two boards that are communicating are four slots apart, at the spacing defined, the backplane length is well defined [as shown].

Note that the Trace Propagation Delay is shown in blue text and the component load capacitance is in red text.
Circuit voltage termination [Vtt] is shown for completeness, but is not involved in the calculation.

Propagation Delay Calculations

Design Hint;
Propagation Delay: 0.8 inches = 2.032cm ~ 0.137nS;
----- Trace Propagation Dealy = 1.0 inch = 2.54cm; Trace Lenght ~ 0.170nS;
----- Backplane Connector ~ 0.135nS
Total prop delay = 1.158nS [Driver to Receiver] = (0.137nS x 4 slots) + (0.17nS x 2 traces) + (0.135nS x 2 connectors)
Trace capacitance is 4.02pF/in [FR4 board material]
Total trace capacitance = 4.02pF x 5.2in = 20.9pF [PWB to PWB distance]
----- This figure changes with board material and outside or internal layer used. [Characteristics of Board Materials]
Total load capacitance = 88.9pF = 20.9pF + (8pF x 5) + (2.8pF x 2)x5 (10 times normal LVCMOS load)
---- Trace capacitance + IC capacitance + connector capacitance
Capacitance per unit load = 17.62pF = 8pF + (2.8pF x 2) + 4.02pF
----- IC output capacitance + connector capacitance
Zmin > 1 / [ 2 x 3.14 x 622MHz x 1.25 x d x Co] = 1 / [2 x 3.14 x 622MHz x 1.25 x 4.57cm x 17.62pF]
----- Minimum impedance
Skew: Connector 8pS/connector [assumes same row]; Backplane 25pS WAG; Board 10pS/board WAG
Total System Skew = 69pS = [8pS x 3] + 25pS + [10pS x 2]
Sample Window = 580pS [Altera data sheet]
TCCS; Channel-to-Channel Skew = 400pS [Altera data sheet, max, -1ver]
TUI; Timing Budget = 1.190nS [Altera data sheet, min, -1 ver]
RSKM = [TUI - SW -TCCS]/2 = [1.190 - 0.580nS - 0.400nS]/2 = 105pS
Margin = 26pS = RSKM - [clock jitter + System Skew] = 105pS - [10pS + 69pS]
CCA to CCA has a 1.158nS trip delay with a 26pS timing window
CCA to CCA adds another 1.158nS trip delay

cPCI Connector Prop Delay
2mm Connector Propagation Delay by Row

Design Hint; Most backplane connector are multi-row devices.
So when doing the above equations be sure to stay within the same row of the connector, otherwise your calculations will be off.
The table above shows the difference in propagation delays depending on the row of pins used.
The higher the row on the [daughter card] connector the greater the propagation delay.
This would be for a right-angle board connector;
a straight connector found on the mother-board would have prop delay rates equal, because all the pins on a straight connector are equal length.

Design Information

Switching Terms -
VCC: The voltage applied to the power pin(s). In most cases the voltage the device needs to operate at.
VIH: [Voltage Input High] The minimum positive voltage applied to the input which will be accepted by the device as a logic high.
VIL: [Voltage Input Low] The maximum positive voltage applied to the input which will be accepted by the device as a logic low.
VOL: [Voltage Output Low] The maximum positive voltage from an output which the device considers will be accepted as the maximum positive low level.
VOH: [Voltage Output High] The maximum positive voltage from an output which the device considers will be accepted as the minimum positive high level.
VT: [Threshold Voltage] The voltage applied to a device which is "transition-Operated", which cause the device to switch. May also be listed as a '+' or '-' value.

Description of TTL, ECL and CMOS Glue Logic Families

Related pages on this site:
Standard Logic Voltage Thresholds Interface Bus Logic Thresholds Glue Logic Logic Speed x Power Chart How to Termination Traces Ground/Power Planes

Back to the Logic Design Page, Also refer to the Delay Logic Manufacturers page.

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Modified: 1/4/12
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