[PIC] Output impedance of PIC18F4550 pin.

[willycat]

Hi,

to reduce reflection and crosstalk, i want to place a serial resistor as close to the output pin of a PIC18F4550 used as a clock signal. But how to determine the output impedance of that pin ?

Is it 0.7/3e-3 = 233 ohms (According to datasheet on page 380, with IOH=-3mA and VOH = VDD-0.7) ?

or 10e-9/50e-12 = 200 ohms (According to datasheet on page 389, TioR = TioF = 10ns when the load is 50 pF) ?

In both cases, i think it is too big, so i am probably wrong.

What is the correct value ?

Thanks,

willy.

 

[KlausST]

Hi,

It simply is not exact and not constant.
The LOW drive may be different from HIGH drive and different from the edges.

Why do you think this is important to know?

Klaus

 

[danadakk]

The values you calculated are from DC values of GPIO pins, not really Z.

The best way to do this is either use a VNA on an output (beware of VNA input
dc range) or simply toggle that output and try different values of R to look at
circuit response. Note if you do not have a VNA you can get NANO VNAs in the
range of $ 40 to $ 80 these days. But you would be doing a static measurement,
and have to pad it when doing Z to supply rail measurements, to handle the limited
DC bias input range of the VNA detection system.

The other problem you have is device to device drive levels vary significantly,
implying Z probably impacted as well. So unless you are going to "swamp" out
the variation with high valued R to dominate your repeatability would be poor.
And of course your clock signal would be affected.

Regards, Dana.

 

[willycat]

Thanks for the replies.

But I didn't know it was so complicated.
Someone tell me that it is a good idea to place a resistor (usually 22 or 33 ohms) in series with a clock line to improve the signal by reducing crosstalk and reflections and to achieve that, it was necessary to properly terminate the line to have at least the source impedance be the same as the transmit line (a pcb track in my case). This is why i wanted to know the output impedance of the pin to determine the correct value for this resistor (look here).

Thanks,

Willy.

 

[danadakk]

What is the clock rate on the pin ? What is it connected to externally ?

Regards, Dana.

 

[FvM]

The quoted datasheet details are specifying maximum voltage drop respectively rise/fall time and thus not suitable to estimate driver impedance. As already stated by others, output driver impedances are non-linear and (in case of TTL compatible ports) not necessarily symmetrical. It's nevertheless possible to derive a typical impedance for the ohmic part of the I/V curve.

Microchip doesn't publish typical I/V characteristics or even IBIS files for the PIC18F family of devices, respectively you need to measure the impedance yourself. I'd guess that PIC18F driver impedance is in a similar range or probably higher than typical single ended PCB trace (e.g. 65 to 75 ohms) and thus don't need additional series termination. Ultimately you'll check with a fast oscilloscope and suitable active or resistive probe.

Said 22 to 33 ohms series termination is a good estimation for CMOS outputs with high drive strength, PIC18F has rather low drive strength.

 

[willycat]

Ok, i will put an eye on this nanovna.

This pin is the clock signal of an SPI interface at approximately 3 MHz directly connected to another chip on the board.

For the serial resistor, i will begin with a 0 Ohm value and adjust it if needed by using an oscilloscope. Is this the way engineers are working ?

Thanks for your help.

Willy.