Re: A7N8X-DLX 2.0 Dimm Regulator Circuit Failure
- From: Paul <nospam@xxxxxxxxxx>
- Date: Fri, 04 Apr 2008 17:45:48 -0400
Kyle wrote:
Thanks Paul, you did not have to go to the effort to find the sample
circuits as I have done such before (fixed my 350W Antec supply
recently using sample circuits and reverse engineering my circuit),
and your effort is appreciated. The other regulator circuit in that
area of the board looks to be providing 5v, so I think it is ok, tho
I'm not certain as to its normal output voltage. Does that sound
right for the second voltage?
No visual heat damage appears on the devices. I can measure resistors
easily, it's a matter of knowing where the resistor resides in a
circuit that determines, of course, what value one should measure, and
having the circuit schematic is critical to know if the resistors are
correct in value. I suppose a schematic is unavailable, as is typical
with mobos.
As to fixing switched mode circuits, I have a scope, variac and dvm
and am able to fix most anything. Agreed, finding the collateral
damage is more challenging, but in this case, the circuits are fairly
simple, and typically the damage done by a bad cap is normally found
in the switching transistors, tho I agree, other components can also
get burned.
My problems all started when I initially started to swap out the PS
from my system, I observed careful static grounding techniques, PS
swap was in fact cancelled once I got it out and reviewed the specs
sticker, I reinstalled same PS (the PS was allegedly a 450w, and I was
moving it to another machine which now has a new vid card that
requires lots of power, but as it turns out, my 350W Antec had better
specs for combined output and 12v rail current capability than the
450w PS, not unusual), then machine would not post. PS outputs
measure fine, PS is not bad as it is running my "other spare" board
now. Funny how a simple operation such as this results in disaster.
When machine would not post, I began inspecting things. Found the
CMOS battery very low, changed the battery, but had trouble with
clearing CMOS as the machine was setup to automatically turn on when
power was applied and this action/feature would not "clear" or reset
when I cleared CMOS, oddly. So the mystery is what caused the vdimm
circuit to fail, machine ran w/o problem prior to the PS
removal/reinstall, so I doubt the vdimm circuit was bad beforehand.
I'm pretty darn certain I turned PS off (switch on back) before
removal and install, and only 1 stick of memory (512meg) is not a load
likely to overheat the caps in the regulator circuit. Frustrated, I
even swapped BIOS chips between my boards (found out the custom FSB
frequency of this system is in the BIOS, no wonder these nforce2
boards have failures with BIOS ROM since FSB info is stored in the
flash ROM when custom FSB's are setup). The machine is never
unplugged, and powered up 24/7, so a bad CMOS battery would not be
discovered until a power down event, such as PS swap, occurred. Oh
well, just another day gone bad in the computer ownership world.
The voltage on the Vcore circuit seems to track the type of CPU I
install, an old Athlon 1400 showed 1.75v, and a newer Applrebred Duron
1600 showed 1.5x volts, so the Vcore circuit seems to setup correctly.
Vdimm regulator circuit appears to be a RT9202 chip, found here:
http://www.richtek.com/www/Docs/DS9202-08.pdf .
I wonder if the Vdimm controller somehow got programmed up wrong
because of CMOS battery failure?
Guess I'll be probing around some to find the answers.
I was thinking, if a MOSFET fails short, you may be able to detect
that with an ohmmeter. By comparison to your powered down, but
still functional motherboard. Maybe. There is a bunch of stuff, like
banks of capacitors, that the meter won't be able to charge
very easily.
The 0.6V is about a diode drop, but that may not have any significance.
These are specs from a DDR datasheet. It appears VTT is 1/2 the supply
voltage. On some boards, that can even be done with a five pin linear
regulator chip. The regulator requirements on that one are special, as
the regulator has to be able to sink some amps or source some amps,
on a cycle by cycle basis. There are usually a bunch of chip caps on that
rail, to smooth out the cycle by cycle high frequency variations, and
give the regulator a chance to respond to the changing current. The
termination type on the memory bus is SSTL (stub series termination),
and the termination current that results is what VTT is handling.
Supply voltage VDD +2.5 to +2.7 V
I/O supply voltage VDDQ +2.5 to +2.7 V
I/O reference voltage VREF = 0.49 × VDDQ to 0.51 × VDDQ
I/O termination voltage (system) VTT = VREF - 0.04 to VREF + 0.04
More info here.
http://en.wikipedia.org/wiki/Stub_Series_Terminated_Logic
http://www.jedec.org/download/search/jesd8-9b.pdf (fig.3 page 14)
With regard to the RT9202, it is a pretty "dumb" chip, and doesn't
take a VID code, like a larger device might. The feedback ratio of
R2 and R3 appears to set the voltage. That could be modified by a
GPIO pin and resistor (if you wanted a second voltage value like 2.7V).
The GPIO pin would be tristated (open circuit) when the board starts,
causing the regulator to start at 2.5V.
The BIOS doesn't need DRAM, to start computing. The BIOS code eventually
gets to the part, where it sets the DDR voltage, if that voltage is
programmable via a GPIO trick. They would enable the GPIO and do whatever
is necessary, once the CMOS setting for Vdimm is consulted. If the CMOS
is corrupt, the CMOS checksum will be wrong, and defaults will be loaded
in their place. The CMOS checksum is not a strong check, so a bogus
set of values could in theory be accepted. But the Vdimm probably has
limits as to how far it can be programmed anyway. Asus are not DFI after
all - they don't generally design in good "burn potential" :-)
http://www.richtek.com/www/Docs/DS9202-08.pdf
And even when a BIOS shows four values for Vdimm, the actual circuit may
only have two values. Just because the BIOS shows them, doesn't mean the
implementation matches the interface.
The RT9202 has overcurrent protection, so it may be able to protect
against some fault modes. It looks like the OC amplifier is looking for
the voltage developed between OC and Phase.
HTH,
Paul
.
- References:
- A7N8X-DLX 2.0 Dimm Regulator Circuit Failure
- From: Kyle
- Re: A7N8X-DLX 2.0 Dimm Regulator Circuit Failure
- From: Paul
- Re: A7N8X-DLX 2.0 Dimm Regulator Circuit Failure
- From: Kyle
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