Does My Circuit Contain High-Speed Signals?

Eyal78

New Member
My circuit includes the following components:
  • PIC16 with internal clock, frequency: 32 MHz
  • ST LED2000 LED driver (a buck converter) with a PWM dimming input
  • TI BQ24070RHL battery charger
  • LED controlled via PWM by the microcontroller.
My question:
Do the following signals qualify as high-speed?
I’m asking this to determine whether I need to ensure a proper return path for them and/or maintain large spacing between them.
If not, can I ignore the return path considerations?

Key signals I believe require attention:
  1. Charger outputs – These are open-drain outputs. These outputs are connected to the microcontroller inputs (with internal PU). I couldn't find rise/fall time specifications in the datasheet.
  2. PWM outputs from the microcontroller– Base frequency: 200–500 Hz.
    • One output goes to the LED driver (possibly with or without a pull-down resistor).
    • Another output drives an LED.
  3. Push button with a pull-down resistor, connected to a microcontroller input.

Thanks in advanced for any answer,
Eyal
 
Since it seems :

1) No external high freqs being fed to pins or incoming present
2) No high current loads

You should have a forgiving design. But you have a 32 Mhz clock dumping
charge into a ground return and its supply pin, so good bypassing, low L
trace runs, good caps (combo bulk and ceramic) on power pin always a
good idea.

Caps technologies and their ESR :



OSCON is polymer tant, bulk.

Dont forget how to handle unused GPIO pins, Microchip has ap note on this topic
and or forum threads.


Regards, Dana.
 
Last edited:
Thanks Dana!
What do you think about PWM output (low 500 Hz, but fast edges of the MCU)?
 
C load, if light, and PCB run to its destination short, should not be a
problem. Does output pin have a speed selection ? Eg. its current capability ?
What is its destination ?

I see this in another datasheet :



Note the NSC ap note on layout target towards switchers, but excellent overall ref on PCB layout.
 

Attachments

  • Chapter12.pdf
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  • Meeting Power Design Challenges 2007 National Semiconductor Seminar.pdf
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Last edited:
In reference to my original question:
It's the PIC16 MCU output (I think not possible to select slew rate, if this is what you mean), The destination is less than 5 cm away to the LED2000 LED driver. Its load is 2uA.
But isn't it a fast signal anyways, requiring a proper ground as a return path, in proximity to the signal? otherwise crosstalk, EMI and other troubles occur?
 

Many PIC's have a slow slew rate option on the I/O pins, however using it for PWM is a bad idea, as you want the switching device to 'switch' and not work in a linear fashion. Slew limited is usually the default option, so is one of the things you need to disable for most uses - for example leaving it ON severely cripples SPI speed.

But mostly, you're massively over thinking this - just get it built, requirements are really pretty low, just apply a little common sense in your design.
 
The dimming input to LERD2000 is a logic input, looks to be buffed by its specs.

So PWM slew rate option, if it exist on your specific part, eg my post #4
table of GPIO selection for a generic part, would be fine and lower the
EMI generated by design. PWM input slew rate does not affect your
design switching control of the LED2000.
 
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