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Visitor anjeo
Visitor
43,007 Views
Registered: ‎02-12-2014

Difference between Slew rate and Drive strength

Hi,

 

There are I/O attributes of slew rate and drive strength.

What is difference between slew rate and drive strength attribute?

 

Usually, it is explained that slew rate can be controlled by drive strength for the load.

Then, what's the concept of that slew rate can controlled seperatly with drive strength?

 

Regards,

Hanseok

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7 Replies
Xilinx Employee
Xilinx Employee
42,998 Views
Registered: ‎02-06-2013

Re: Difference between Slew rate and Drive strength

 

Hi

 

Have a look at the below links for the definition of drive strength and slew rates which apply to all the devices.

 

http://forums.xilinx.com/t5/7-Series-FPGAs/What-does-the-quot-output-slew-rate-quot-mean/td-p/413467

 

http://forums.xilinx.com/t5/7-Series-FPGAs/Kintex-7-Output-Drive-Strength-definition/td-p/412321

 

Hope it helps,

Regards,

Satish

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Xilinx Employee
Xilinx Employee
42,990 Views
Registered: ‎08-01-2012

Re: Difference between Slew rate and Drive strength

Slew rate help to set the desired  the rise and fall times of output

Drive stregth help to set the desired  output current  to meet the load requirements.

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Visitor anjeo
Visitor
42,981 Views
Registered: ‎02-12-2014

Re: Difference between Slew rate and Drive strength

Thanks but my question is : how can the attribute "slew rate" be treated seperatly with the attribute "drive strength"?

 

Usually, for steeper slew rate, more drive strength is needed,

for more gradual slew rate, weaker drive strength is required.

(based on I = C*dV/dt )

 

 

 

 

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Xilinx Employee
Xilinx Employee
42,971 Views
Registered: ‎08-01-2012

Re: Difference between Slew rate and Drive strength

Slew rate is mainly meant for timing requirements. The slew rate selction can be done while setting IO standard in ucf file.

 

With higher loads (Load capacitance)  the rise/fall times increases but not much related to drive stregths.

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Historian
Historian
42,921 Views
Registered: ‎02-25-2008

Re: Difference between Slew rate and Drive strength


@anjeo wrote:

Thanks but my question is : how can the attribute "slew rate" be treated seperatly with the attribute "drive strength"?

 

Usually, for steeper slew rate, more drive strength is needed,

for more gradual slew rate, weaker drive strength is required.

(based on I = C*dV/dt ) 


Drive strength lets you accomodate heavier or lighter loads. As you increase the load (which at high-speed digital switching speeds is mostly capacitive), you need to be able to source enough current fast enough to charge or discharge the load capacitance. This is why ancient TTL databooks would refer to output drive capability in "number of TTL loads." Not enough drive strength and your signal may not reach a valid logic level when it needs to be valid (by the next clock edge, for example). Too much drive strength and the signal may ring (overshoot/undershoot), which is a signal-integrity problem and can lead to double-clocking and other invalid logic conditions.

 

Drive strength can be related to source impedance, which is important when matching to trace impedance.

 

Slew rate is a large-signal property of amplifiers and drivers, and indicates the rate of change of the output from its lowest possible level to the highest (or the converse). If your slew rate is too slow, then as above your signal may not reach a valid logic level when it needs to. Slew rate is defintely taken into account by the timing analyzer tools.

 

So basically you want the "correct" drive strength based on load and the "correct" slew rate based on output toggle rate.

----------------------------Yes, I do this for a living.
Instructor
Instructor
42,908 Views
Registered: ‎08-14-2007

Re: Difference between Slew rate and Drive strength

Just to add one point to bassman's excellent explanation, when your load is mostly capacitive (this is typical of modern CMOS circuits) the drive strength will affect the actual slew rate.  The slew control parameter (QUIETIO, SLOW, FAST) limits the slew of the on-chip drivers.  That sets the fastest rate at which the drivers will slew with no output load.  Adding capacitance will slow down the slew rate.  Driving a capacitor you typically use the equation:

 

C * V = I * T

(capacitance times voltage = current times time)

note that this should really be a differential equation, but it works for constant current.

 

Solve that for slew rate:

 

V / T = I / C

 

So this says that drive-limited slew rate goes up linearly with drive current, I, and as the inverse of load capacitance C.

 

In the old TTL days, drivers were usually limited to a small number of loads due to the DC current requirements of the load.  With CMOS, you can drive any number of loads if you have enough time to wait for all that capacitance to charge up and down.  Here adding drive strength can increase the capacitive load capability at any given bit rate, or increase the bit rate into any given capacitance.

-- Gabor
Observer a@1
Observer
5,406 Views
Registered: ‎04-17-2018

Re: Difference between Slew rate and Drive strength

Hi Team,

 

What is the numerical value for Slow slew rate and fast slew rate ?

 

Thanks

Abhilash

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