Hi,
I would like to know what SLEW RATE is.

I'm using a PIC 18F46K20 and controlling a TFT screen using SPI.

Thanks, C.

 

Slew rate is about the rising and falling speed of a signal.
Most times it is used with opamps.

https://en.m.wikipedia.org/wiki/Slew_rate
https://www.electrical4u.com/slew-rate/

Bertus

 

Slew rate is about the rising and falling speed of a signal.
Most times it is used with opamps.

https://en.m.wikipedia.org/wiki/Slew_rate
https://www.electrical4u.com/slew-rate/

Bertus

Hi B,
I see !
I presume this applies to anything with a square wave, such as SPI signals.

I'm trying to figure out a PIC/TFT intermittent problem, where the TFT sometimes doesn't fire, but the PIC is still running.
Could it be cloned PIC or an error in the program/er or even a problem with the PCB?
C.

 

There may be problems when a pulse is to slow.
The transition from high to low or low to high might take to long when the slew rate is to low.

Bertus

 

There may be problems when a pulse is to slow.
The transition from high to low or low to high might take to long when the slew rate is to low.

Bertus

Hi B,
Ok, thanks.
I'll carry on trying to solve this.
C.

 

Even a capacitive load can cause problems.
The load can slowdown the transition speed.

Bertus

 

Hi,
I would like to know what SLEW RATE is.

The slew rate SR defines how fast the output of an amplifier with feedback is able to follow the corresponding input signal.
However, we have clearly distinguish between (1) small signal rise time and (2) large signal behaviour.

(1) The small-signal rise time depends on the small-signal bandwidth of the amplifier only and is defined also for amplfier stages without feedback.

(2) In contrast, the slew rate SR is defined for feedback amplifiers like opamps only.
It can be measured with heavy feedback applied (usually for closed-loop unity gain) and a square wave input signal that is able to bring the 1st stage of the opamp into saturation (feedback is not active due to unavoidable delays within the amplifier) - until the feedback effect brings the whole amplifier back to linear amplification.
As a consequence, in the first short time period the opamps first stage acts as a current source which loads the compensation capacitor of the 2nd stage. This effect causes the characteristic linear slope of the output signal caused by a the slew rate which is too small to follow the input signal.
This linear slope of the output defines the slew rate given in V/µs.

Comment: Any other effects (caused by a capacitive load, for example) have nothing to do with the slew rate.

 

Even a capacitive load can cause problems.
The load can slowdown the transition speed.

Bertus

Hi B,
I agree, although I don't know much about this, is there a simple test?
C

 

The slew rate SR defines how fast the output of an amplifier with feedback is able to follow the corresponding input signal.
However, we have clearly distinguish between (1) small signal rise time and (2) large signal behaviour.

(1) The small-signal rise time depends on the small-signal bandwidth of the amplifier only and is defined also for amplfier stages without feedback.

(2) In contrast, the slew rate SR is defined for feedback amplifiers like opamps only.
It can be measured with heavy feedback applied (usually for closed-loop unity gain) and a square wave input signal that is able to bring the 1st stage of the opamp into saturation - until (due to unavoidable delays within the amplifier) the feedback effect will bring the whole amplifier back to linear amplification.
As a consequence, in the first short time period the opamps first stage acts as a current source which loads the compensation capacitor of the 2nd stage. This effect causes the characteristic linear slope of the output signal caused by a the slew rate which is too small to follow the input signal.
This linear slope of the output defines the slew rate given in V/µs.

Hi L,
As I don't know the ins and outs of the TFT, I don't know whether it has opamps or not, but SR has been suggested as a possible problem.
C

 

Hi L,
As I don't know the ins and outs of the TFT, I don't know whether it has opamps or not, but SR has been suggested as a possible problem.
C

Hi camerart, I am only able to give you the definition (and its theoretical background) for the large-signal parameter SR (slew rate). I cannot know If this parameter has any relevance for your application.

Question: What is the form of the output signal for a (ideal) squarewave input?

 

hi C,
Which TFT model, PIC, clock rate?
E

 

Hi camerart, I am only able to give you the definition (and its theoretical background) for the large-signal parameter SR (slew rate). I cannot know If this parameter has any relevance for your application.

Question: What is the form of the output signal for a (ideal) squarewave input?

Hi L,
I don't know how to answer your question, sorry.
C

 

hi C,
Which TFT model, PIC, clock rate?
E

Hi E,
There are 2x TFTs on is ST7789
I'm using 18F46K20
We are changing the clock rate, until we get consistent result.

The question regarding slew rate is for my education, I have tested stuff and seen sloping square waves, and now I think I know what it is called.
C

 

For digital edges, it's normally referred to as their Rise and Fall times, rather than slew rate.

 

Sometimes imagery is useful in concepts for a visual thinker. I find it interesting that geometry
can help some to adapt a mathematic expression and gain confidence from past fear or math anxiety.
While some are struggling with algebra others are playing with numbers writing proofs not needing much geometry.
Others can visualize an animation of a gradual transformation in order to formulate a rate of change.
The picture below shows voltage over time. The formula shows voltage over time. As the numbers flow
through the Slew rate formula a change takes place as it approaches the Maximum slew rate.
With a periodic wave there is frequency. One of the Slew Rate characteristics is the frequency limit.

The expression for Slew Rate is



Have you ever wondered what electrical condition affects a sine wave to become more triangular?
Have you ever asked why a square wave starts to change shape to trapazoidal?
Good that is Slew Rate otherwise we chance opposition talking Geek to a Greek in figure b) ∀≠∃

 

Sometimes imagery is useful in concepts for a visual thinker. I find it interesting that geometry
can help some to adapt a mathematic expression and gain confidence from past fear or math anxiety.
While some are struggling with algebra others are playing with numbers writing proofs not needing much geometry.
Others can visualize an animation of a gradual transformation in order to formulate a rate of change.
The picture below shows voltage over time. The formula shows voltage over time. As the numbers flow
through the Slew rate formula a change takes place as it approaches the Maximum slew rate.

The expression for Slew Rate is View attachment 340373

Have you ever wondered what electrical condition affects a sine wave to become more triangular?
Have you ever asked why a square wave starts to change shape to trapazoidal?


View attachment 340370

View attachment 340371

Hi S,
Ok, that's clear.
C

 

For digital edges, it's normally referred to as their Rise and Fall times, rather than slew rate.

Hi S,
When others were trying to trouble soot the problems, slew rate was mentioned, so this is why.
C

 

Hi C,
Read this link data, explains the edge rise/fall timing limits etc...
E
https://www.picotech.com/library/knowledge-bases/oscilloscopes/rise-time

 

Hi C,
Read this link data, explains the edge rise/fall timing limits etc...
E
https://www.picotech.com/library/knowledge-bases/oscilloscopes/rise-time

Hi E,
Thanks.

Actually the full problem is here: https://forum.allaboutcircuits.com/posts/1961000/
Someone posted that the problem could be 'slew rate' this is why I asked, but it may not solve the problem, and I'm trying to figure out the possibilities.
Cheers, C