Maker Pro
Maker Pro

0.0 ohm resistor??

A

Ant_Magma

Jan 1, 1970
0
Guys, i'm currently working on National Semiconductor's DP83847
Ethernet Transceiver.

In their reference designs, there's this 0.0 ohm resister at the output
pin of the 25Mhz oscillator

http://geocities.com/Antonio_Magma/0resistor.gif

Is there such thing as a 0 ohm resistor? How do i even build it? Just
ignore it and place a wire?
 
P

Pooh Bear

Jan 1, 1970
0
Ant_Magma said:
Guys, i'm currently working on National Semiconductor's DP83847
Ethernet Transceiver.

In their reference designs, there's this 0.0 ohm resister at the output
pin of the 25Mhz oscillator

http://geocities.com/Antonio_Magma/0resistor.gif

Is there such thing as a 0 ohm resistor? How do i even build it? Just
ignore it and place a wire?

It's a 'dummy' component. Maybe it was originally inteneded to have a value
but it was found unnecessary.

Graham
 
P

PeteS

Jan 1, 1970
0
Ant_Magma said:
Guys, i'm currently working on National Semiconductor's DP83847
Ethernet Transceiver.

In their reference designs, there's this 0.0 ohm resister at the output
pin of the 25Mhz oscillator

http://geocities.com/Antonio_Magma/0resistor.gif

Is there such thing as a 0 ohm resistor? How do i even build it? Just
ignore it and place a wire?

As Graham noted, it could well be a dummy component. In some designs,
the chip mfrs are not completely clear in their datasheets and there
are times I will drop in a zero ohm resistor as a placeholder for some
other value if I need to use it (drive limit to a crystal is a pretty
common use).

Another place I have used them is where I need a shorted link, but
which way around is in doubt. I have (in one of my designs) a resistive
touch panel and I use optional zero ohm resistors to make sure I get
the directional sense correct (even if the manufacturer changes the
panel).

There are myriad other uses, of course.

In your case, replace it with a wire.

Cheers

PeteS
 
B

Bob Masta

Jan 1, 1970
0
It's a 'dummy' component. Maybe it was originally inteneded to have a value
but it was found unnecessary.

Another use for 0 ohm resistors is to act like a jumper.
They are better than the little wires you might bend
up yourself for a one-off, since automatic insertion
equipment can handle them just like regular resistors.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator
 
P

Pooh Bear

Jan 1, 1970
0
PeteS said:
As Graham noted, it could well be a dummy component. In some designs,
the chip mfrs are not completely clear in their datasheets and there
are times I will drop in a zero ohm resistor as a placeholder for some
other value if I need to use it (drive limit to a crystal is a pretty
common use).

Another place I have used them is where I need a shorted link, but
which way around is in doubt. I have (in one of my designs) a resistive
touch panel and I use optional zero ohm resistors to make sure I get
the directional sense correct (even if the manufacturer changes the
panel).

There are myriad other uses, of course.

In your case, replace it with a wire.

Agreed.

In a cost-sensitive applications, a 'zero ohm resistor' or link may be used
to 'cross tracks' in order to make possible a 'single-sided' printed circuit
board in place of a double-sided one.

Much low cost consumer equipment uses single sided pcbs.

It's rare to see such links identified as components though.

Graham
 
A

Ant_Magma

Jan 1, 1970
0
Thx 4 ur replies guys. I'll replace them with wires.
 
C

Chris

Jan 1, 1970
0
Ant_Magma said:
Guys, i'm currently working on National Semiconductor's DP83847
Ethernet Transceiver.

In their reference designs, there's this 0.0 ohm resister at the output
pin of the 25Mhz oscillator

http://geocities.com/Antonio_Magma/0resistor.gif

Is there such thing as a 0 ohm resistor? How do i even build it? Just
ignore it and place a wire?

Hi, Ant. It would have been good if you'd indicated a link to the
National reference design -- your external oscillator driver isn't in
the data sheet or the first couple of app notes on the National
website.

http://cache.national.com/ds/DP/DP83847.pdf
http://www.national.com/appinfo/networks/files/dp83847_IBM_ppc440gp.pdf

The IC either uses a crystal oscillator or an external oscillator. The
data sheet says that, for the external oscillator, drive X1 directly,
and leave X2 open. So, ignore the 0.0 ohm resistor, and just connect
the output of the external oscillator to X1.

I'd guess IBM just needed a jumper on their board.

It's easier to get help if you show us where to look.

Good luck
Chris
 
A

Ant_Magma

Jan 1, 1970
0
Ah, thx Chris. I'll keep that in mind.

Since u mentioned about leaving X2 open this leads to my next question.

1) In the description on the datasheet, the X2 is described as the
primary clock reference output and this pin isn't used. Does it mean
that this pin outputs a 25mhz reference frequency?

2) From the reference designs in the link by Chris (IBM), some of the
pins at the MII interface have a 49.9ohm resistor some dont. However,
in the reference design by National, they are replaced with 0.0ohm
resistors. Which is which? What purpose does this resistors serve? Does
it mean that it's optional (i want to save PCB space without the
resistors)?

3) If i have other components that requires 25mhz oscillator, is it ok
to share it with this chip? Will it pose any inteference problems? Or
must every difference component have their own oscillators and not
share?

Thx guys...
 
M

Michael A. Terrell

Jan 1, 1970
0
Pooh said:
In a cost-sensitive applications, a 'zero ohm resistor' or link may be used
to 'cross tracks' in order to make possible a 'single-sided' printed circuit
board in place of a double-sided one.

Much low cost consumer equipment uses single sided pcbs.

It's rare to see such links identified as components though.


Bull. It IS a component, because it has to be on the BOM, and if its
surface mount, it has to be loaded on the P-N-P machine, or placed by
hand. Jumpers, on the other hand are classes as wires, not resistors.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
C

Chris

Jan 1, 1970
0
Ant_Magma said:
Ah, thx Chris. I'll keep that in mind.

Since u mentioned about leaving X2 open this leads to my next question.

1) In the description on the datasheet, the X2 is described as the
primary clock reference output and this pin isn't used. Does it mean
that this pin outputs a 25mhz reference frequency?

2) From the reference designs in the link by Chris (IBM), some of the
pins at the MII interface have a 49.9ohm resistor some dont. However,
in the reference design by National, they are replaced with 0.0ohm
resistors. Which is which? What purpose does this resistors serve? Does
it mean that it's optional (i want to save PCB space without the
resistors)?

3) If i have other components that requires 25mhz oscillator, is it ok
to share it with this chip? Will it pose any inteference problems? Or
must every difference component have their own oscillators and not
share?

Thx guys...

Hi, Ant. If you look at the data sheet, you'll see (view in fixed font
or M$ Notepad):

|
| | |
| | |
| | |
| | |
| | |
| | |
| | X1 X2 |
| '--o-------o---'
| | |0.0 ohms
| | .-.
| | | |
| | | |
| | _ '-'
| | | | |
| o-|| ||-o
|22pF| |_| |22pF
| --- ---
| --- ---
| | |
| === ===
| GND GND
|
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

On p.26, it shows that the X2 output is necessary to feed energy back
into the crystal for oscillation. Don't connect X2, no oscillation. R
can be 0.0 ohms (for the type of crystal specified in the data sheet).
Long story.

Anyway, this is a typical setup for a crystal-based clock on
microcontrollers. The business behind the chip is described in
numerous places, particularly Don Lancaster's CMOS Cookbook. He
doesn't get into the math. For a somewhat more detailed
non-mathematical explanation, see Chapter 5 of Art of Electronics
(Horowitz/Hill).

Unless it specifically says so in the processor/uC data sheet, NEVER
use X2 for anything except oscillator feedback. You're loading down
the oscillator, and you could easily inhibit oscillation unless you do
it right.

I don't have experience with your chip. I'd really recommend you
download all of the information at National (datasheet, appnotes,
design guide, &c) and go through it carefully. Your questions kind of
indicate you haven't done that, yet. I believe from quick scanning
that your answers to 2) is there somewhere, relating to changes if
you're using the chip for high-speed operation. The answers are almost
always in the documentation provided with the chip -- they do know what
they're doing, and they don't want to make it any harder to use the
chip.

If neither this or other s.e.b. responses are sufficient, try
sci.electronics.design. Someone there may have designed in the chip.
Make sure you're up to speed on what you're doing, first.

Good luck
Chris
 
P

Pooh Bear

Jan 1, 1970
0
Michael A. Terrell said:
Bull. It IS a component, because it has to be on the BOM, and if its
surface mount, it has to be loaded on the P-N-P machine, or placed by
hand. Jumpers, on the other hand are classes as wires, not resistors.

It's rare to see it identified as a component on a schematic , as per the OP's
link.

I've seen BOMs where such links are accounted for simply by a single length of
tinned copper wire equal to the total use too.

Graham
 
A

Ant_Magma

Jan 1, 1970
0
Thx Chris, i've already read the datasheets and the reference designs.
I'm not lazy, i'm just new.

I'm confused becoz the datasheet says to connect the oscillator to X1
and leave X2 open. And yet in the circuit its connecting both X1 and X2
like you showed in your post. So which is which?
 
Ant_Magma said:
Thx Chris, i've already read the datasheets and the reference designs.
I'm not lazy, i'm just new.

I'm confused becoz the datasheet says to connect the oscillator to X1
and leave X2 open. And yet in the circuit its connecting both X1 and X2
like you showed in your post. So which is which?

Do you have an oscillator (power, ground, out) or just a crystal (two
leads)?

With the oscillator option you connect only one pin to the chip, with
the crystal you connect two because the active part of the oscillator
circuit is inside the chip.

If you want to share the clock and are using an external oscillator
(one pin to the chip) you could use a buffer to do so. You might even
get away with simply using a gate from a hex inverter or quad nand gate
to seperately clock eached driven chip.
 
D

DJ Delorie

Jan 1, 1970
0
Ant_Magma said:
I'm confused becoz the datasheet says to connect the oscillator to X1
and leave X2 open. And yet in the circuit its connecting both X1 and X2
like you showed in your post. So which is which?

Most likely you've got a board that's designed to be used multiple
ways, with the zero ohm resistors acting like configuration jumpers.
Populate resistor for configuration X, don't populate for
configuration Y.
 
A

Ant_Magma

Jan 1, 1970
0
Ic, so i have 2 options with my crystal oscillator depending on whether
i want to use just the crystal or a oscillator.
 
M

Michael A. Terrell

Jan 1, 1970
0
Pooh said:
It's rare to see it identified as a component on a schematic , as per the OP's
link.

I've seen BOMs where such links are accounted for simply by a single length of
tinned copper wire equal to the total use too.

Graham


You must be stuck in the thru hole world. 0 Ohm resistors are common
in RF designs, and every surface mount board RF I've worked on has had
them designated as a component. I saw a lot of them on the digiatal
boards, as well.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
P

PeteS

Jan 1, 1970
0
Ant_Magma said:
Thx Chris, i've already read the datasheets and the reference designs.
I'm not lazy, i'm just new.

I'm confused becoz the datasheet says to connect the oscillator to X1
and leave X2 open. And yet in the circuit its connecting both X1 and X2
like you showed in your post. So which is which?

This is very common.

X1 and X2 are connected (internally) to an oscillator amplifier.

if you are using a crystal, then the crystal is connected across X1 and
X2. Alternatively, *if you happen to have a 25MHz clock available and
wish to use it for this chip*, then connect the clock (from somewhere
else in the system) to X1 and leave X2 open.

In terms of the device, X1 is the input to the oscillator, X2 is it's
output.

So if you are using a crystal, connect it as shown.

Here's what the datasheet says:
(P25 in acrobat reader. P26 of datasheet)

[Highlights mine]
3.7 Crystal Oscillator Circuit
The DsPHYTER II supports an external CMOS level oscillator source or a
crystal resonator device. *****If an external clock source is used, X1
should be tied to the clock source and X2 should be left
floating******. In either case, the clock source must be a 25 MHz
0.005% (50 PPM) CMOS oscillator or a 25 MHz (50 PPM), parallel, 20 pF
load crystal resonator.
Figure 10 below shows a typical connection for a crystal resonator
circuit. The load capacitor values will vary with the crystal vendors;
check with the vendor for the recommended loads.
The oscillator circuit was designed to drive a parallel resonance AT
cut crystal with a minimum drive level of 500µW and a maximum of 1mW.
If a crystal is specified for a lower
drive level, a current limiting resistor should be placed in series
between X2 and the crystal.
As a starting point for evaluating an oscillator circuit, if the
requirements for the crystal are not known, CL1 and CL2 should be set
at 22 pF, and R1 should be set at 0Ω.

Cheers

PeteS
 
R

Rich Grise

Jan 1, 1970
0
It's a 'dummy' component. Maybe it was originally inteneded to have a
value but it was found unnecessary.
Another use for 0 ohm resistors is to act like a jumper. They are better
than the little wires you might bend up yourself for a one-off, since
automatic insertion equipment can handle them just like regular resistors.[/QUOTE]


http://www.irctt.com/pdf_files/ZEROHM.pdf

Cheers!
Rich
 
Top