Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
Paul wrote:
> In article <crkmg01ung612vpra5bplsh7o8de8crldc@4ax.com>, JK
> <junk@mailzone.com> wrote:
>
>
>>On Sat, 31 Jul 2004 09:18:35 +0200, geos <geos@nowhere.org> wrote:
>>
>>
>>>hello,
>>>
>>>I'm going to make some modifications to my motherboard as described on:
>>>http://tipperlinne.com/p2b-ds150.htm
>>>
>>>I have a question about 10K ohm resistor. There is used the term "The
>>>10K ohm *programming resistor*". Since I'm not a professional I would
>>>like to ask if this term ("programming resistor") refers to the resistor
>>>of special type or not?
>>>
>>>Would it be OK if I buy just "ordinary" 10K ohm resistor or I should
>>>look for some specific one?
>>>
>>>I would appreciate any response that will clarify that to me. I don't
>>>live in US and I have to translate all terms used on the page mentioned
>>>into local ones.
>>>
>>
>>In the shop: ask for FS3 programming resistor ! and see if they have
>>that and buy a: p2b soldering iron
>>
>>Just an idea
>>
>>best regards
>>
>>John
>
>
> Nyuk Nyuk Nyuk!
>
> Of course it is an ordinary resistor. You can use any of several types,
> whatever is available at your local electronics store. The value isn't
> that critical. Like 5% is OK, 1/8W or 1/10W or even 1/20th of a watt
> are safe power ratings. The only detail, is the form of the resistor,
> either surface mount or through hole (axial leaded type). The resistors
> on the motherboard are surface mount, and for this application SMT would
> be the best if you can find some.
>
>
http://www.digikey.com (Enter P10.0KHCT-ND in the search box
> The resistors are 10 for $0.90)
>
http://dkc3.digikey.com/PDF/T042/0914.pdf (link valid until catalog update)
>
> The way this resistor is being applied, means it is named "programming
> resistor" or a "strapping resistor". The idea is, a pin on an IC can be
> used for two purposes, where one purpose exists at "T=0 seconds" and
> the other purpose exists after that.
>
> Let us draw a circuit...
>
> +3.3V
> +3.3V ---X |
> 10K |
> X----/\/\/\---+ \ (Internal
> "strap" | / 240K
> GND ---X | \ Pullup
> ------------- | / Resistor)
> | | |
> | | | 22 ohm
> PCICLK0 |----+--+--/\/\/\---------> Clock to PCI slot
> | Series
> | Damping
> ------------- Resistor ("R99")
>
> When the circuit is running in its normal mode, PCICLK0 will be
> enabled as an output. A square wave at 33MHz would be coming out
> of that pin. The purpose of the 22 ohm resistor, is to match the
> load at the end of the line, which in this case would be a PCI
> plugin card. The sum of the output impedance of the pad, plus the
> 22 ohm resistor, is attempting to match the transmission line
> impedance of the clock track. So, this is all that is needed of
> the circuit, to do the normal stuff...
>
> -------------
> |
> | 22 ohm
> PCICLK0 |----------/\/\/\---------> Clock to PCI slot
> | Series
> | Damping
> ------------- Resistor ("R99")
>
> Now, the other condition to worry about, is what happens at powerup,
> i.e. when time is at zero seconds in the life of the IC. The PCICLK0
> pin is not enabled to drive out at that time, and is instead an input.
> It can sense a logic "1" as a 3.3V level, or a logic "0" as a 0V
> level. The chip actually has a resistor of value 240K ohms inside
> the chip, and the end of the resistor goes to +3.3V. This places a
> weak 3.3V level on the pin, and that is the reason that no FS3 circuit
> is required. So, with no modification to the circuit, FS3 always sees
> a logic "1" caused by the weak (high resistance) internal resistor.
>
> The high resistance is important. When the PCICLK0 is driving its square
> wave, which is most of the time, the driver is so strong, it can
> "override" the value the 240K pullup resistor was making at time t=0.
> And, that is the trick of using "strapping" resistors, is the values
> of the resistors are selected to "overcome" one another.
>
> Our friend "P2B" has selected a 10K ohm resistor for a reason. It
> is a stronger resistor than the 240K ohm resistor inside the chip.
> When the 10K ohm "strap" is added to the circuit, and the other end
> of the resistor goes to either +3.3V or GND, the 10K ohm resistor can
> override what the 240K ohm resistor is doing. So, when a jumper plug
> is used to connect the left hand end of the 10K ohm resistor to GND,
> for example, the 10K resistor is saying to the PCICLK0 pin, "I want
> a logic zero please". Since the 10K resistor is strong compared to
> the 240K resistor, the 10K resistor "wins". There is actually a voltage
> divider action, but I'm trying not to get too technical here.
>
> But, of course, the PCICLK0 driver is much stronger than either the
> 10K ohm "strapping" resistor, or the 240K pullup resistor inside the
> chip, so when the chip is finished initializing, the PCICLK0 will drive
> out its square wave clock signal, and the other resistors just
> "go along for the ride".
>
> A detail that wasn't pointed out on the web site, is that another
> function of the 10K ohm resistor, is it is separating the high
> frequency part of the circuit (the "square wave"), from those nasty
> long wires used to connect to the three pin header which is hot melt
> glued to the PCB. It is important that the 10K ohm resistor be fastened
> as close as possible to the 22 ohm resistor. The best resistor for
> that purpose, would be a surface mount (SMT) resistor, of size
> 0603 or so.
>
> This wire can be long.
> |
> |
> v 10K
> X----/\/\/\---+
> "strap" |<------ Keep this wire physically short,
> | as it can upset the high frequency
> ------------- | signal.
> | |
> | | 22 ohm
> PCICLK0 |----+-----/\/\/\-----
>
> Surface mount resistors are not a lot of fun to work with. They
> tend to stick by surface tension, to the tip of the soldering iron.
> In this case, an SMT resistor could be reflow soldered to the
> existing junction where "R99" is soldered to the motherboard.
>
> Most hobbyists will have access to the older through hole resistors,
> the axial ones with the wires sticking out of the end. These should
> not be soldered directly to the board! The problem is, the wire on the
> resistor is stiff, and after soldering is finished, if you accidently
> put any stresses on the resistor (tugging), you can actually rip a
> copper pad off the motherboard. I recommend using some thin wire,
> such as 30 gauge wire wrap wire (kynar insulation is easier to strip
> than teflon). The purpose of the thin wire, is it bends easily and
> that wire will "give" first, rather than the copper pad.
>
> The max stub I would have off that circuit, is perhaps 1/2". That
> is the length of wire plus the lead on the resistor, as shown
> in the above figure with the "Keep this wire physically short"
> arrow next to it. That is why soldering an SMT resistor directly
> to R99, keeps the equivalent wire length to well under 0.1".
>
> If you feel these conditions are too difficult to meet, the
> only downside of using longer wires at the 22 ohm resistor, is
> that whatever PCI slot or PCI device is driven by PCICLK0, may
> get misclocked. Simply not using the PCI slot it connects to is
> one solution. However, if the PCICLK0 signal is used to clock
> the Southbridge, then you must maintain the high frequency
> quality of the signal, if you expect the board to boot.
>
> The datasheet for the clock gen goes over some of these details.
>
>
http://www.icst.com/products/pdf/ics9250-08.pdf
>
> So, now you know the "secret" of strapping ICs. Many ICs use
> this same principle, of using output pins as inputs at time
> t=0, and latching or sampling the value "strapped" to the pin
> for that short time. As long as the "strapping resistor" is weak,
> the normal output circuit never knows it is there.
>
> HTH,
> Paul
Great post, Paul!
I always use a surface mount resistor because it's much easier to solder
directly to R99 than any other type - besides, they are plentiful and
free if you have a few parts boards lying around. I wet R99 with
no-clean liquid flux, hold the resistor (with tweezers) on top of R99 at
about a 30 degree angle, reflow the solder, then solder 30AWG wire-wrap
to the 'upper' end of the new resistor, and finally add a drop of
hot-melt glue for mechanical protection.
I've had email discussions with several modders who insist on installing
the programming resistor at the jumper instead of at R99 because it's
easier. It seems to work, but I won't do it for the reasons you describe
- if you have problems with PCI devices later, diagnosing a bad PCI card
vs. a degraded PCI clock signal could be a nightmare!
P2B