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Rigol DP832 Random Reboots

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T2:
Hi there,

I know this thread is quite old.

Myself, I'm looking at a dp832 that has "developed" this reboot issue over time.
My Rigol has the v2.10 (2013-03-26) top board (with the large heat sink).

- the digital load seems quite "dynamic", the current comsumption is not stable but "jumps around"
- using an external 5V power source for the digital supply just intensivies the problem (Minutes instead of hours/days)
  -> longer cables, higher impedance

- Measuring the voltage with an oscilloscope (AC coupled) shows sporadic, rather high spikes/bursts
- >1.2Vpp (measured with the built-in 5V supply) !!

This leads me to believe that instabilities/noise on the 5V rail may be culpable for the spontaneous reset/reboots fo the DP832.
My next palnned steps are to dismantle the logic board (front board) and look there closer.
- Caps going bad?
- Insufficient decoupling close to the MCU or reset logic?


/Thomas

T2:
Dug further down. Some of my previsouly observed 5V spikes may have been coupled in by my measurement setup.

The digital front board seems to be really a finicky thingie. I came across an 1117 linear regulator (5V in, 3.8V out, labeled "Battery") that basically 1:1 passed the input ripple from the 5V line to the output. With a maximum drop-out voltage of (according to datasheet) 1.1 to 1.3V, this seems a bit on the close side (5V supply has around 5.2V, so deltaV is 1.4V). There are 3 ceramic output caps present (see schematic fragment) but I don't trust them.

The other supply voltages (3.3V, 1.8V, etc) look more or less OK but have quite some digital noise on them.

I added an input (22u, yeah I rounded up) and an output cap (100u) to the 3.8V regulator as the datasheet sugegsts. The output voltage then became rather stable, not passing on any ripple from input. Check the pictures for my hack.

I also noted that the CPU gets rather hot (you wouldn't let your finger there for long time), so I put a small heatsink on.

Now starts the waiting part...






EDIT: Still rebooting. Need to look further.

T2:
Hi there,

I think I finally fixed it. My DP832 is now running without reboot for more than 24h.

If you have a Rigol DP832 with a DigitalBoard V02.00 2012-12-07 (front-side board with MCU)
and your Rigol is not doing sporadic reboots you may just be lucky.

TopBoard: v2.10 (large heat-sink for 5V regulator)
DigitalBoard V02.00

My DP832 started to develop the sporadic reboots over time, from once a month to about once a
day to multiple times a day by now. Let's face it - unusable as a bench supply. And yes, my DP832
is running 24x7 - but that's something you'd expect from a bench-supply.
Some days ago I dismantled my Rigol and started poking around on the digital board.

To summarize, aside from the linear 5V regulator/toaster on the top board that we all know about,
I identified 2 issues while checking the voltage rails on the digital front board.


The "Battery" rail:
The AC noise from the 5V rail was passed straight through to the output of the regulator (1117 ADJ).
I added input and output caps (although there are some ceramic caps on the output side, but I
don't trust them). Also the deltaV (Vin-Vout) was very close to the dropout voltage. My guess is that
this is the main reason for the ripple being passed straight through (the regulator was not regulating
anymore).

Studying the iMX28 (Application processor / MCU) datasheet a bit, shows, that this beast
is designed to be powered from two power sources. A 5V source as well as a Li-ion battery
source (3.1 ... 4.2V). The MCU automatically switches between those sources.


The "VDD4P2" rail:
This is an intermediate power rail (4.2V) from the MCUs integrated switching supply. I did
measure a *sh%@tload* of switching ripple (>2Vpp) on this rail. According to the reference
schematic for the iMX28, there *must* be a handful of decoupling caps installed on this rail.
However, by just quickly probing around I could not find any serious caps (just one tiny 0402 cap)
on the Rigol board (I don't say there are no caps, just say that I did not find any - hard to tell
for sure  without schematic). Luckily this rail is accessible from the back side of the board, no
need to disassemble the front.
I added some caps (ceramic and electrolyte) to this rail and the switching noise got *way* better.

I'm still measuring up to 400mVpp switching noise, but stability has improved massively.
Before this modification, with the voltage reduced to 4.9V on the 5V rail, I got multiple reboots
per minute. After adding the caps it just kept running. No reboots observed. :-)

So my guess is: fix #2 is actually the one that cures the spontaneous reboots. Problem #1
probably has no direct impact on that (depends maybe when/if the MCU actually takes
the power feed from the "Battery" rail - if at all).


A lower voltage an the 5V rail causes the reboots to happen faster / more frequent.
My guess is: lower voltage -> higher currents (switching supply in the iMX28) -> more switching ripple on 4P2 rail
-> at some point the MCU resets (likely the built-in reset circuit or brown-out detection triggers)

This could even explain why it happens more frequent on devices with the small heat-sink (old top board).
If the LM317 starts to cut back due to thermal conditions -> lower voltage -> higher probability for reboot

Note: This VDD4P2 rail issue might me systematic! A cross check with a properly working (no reboots)
DP832 of a friend (same DigitalBoard rev) did show massive ripple of close to 2Vpp on this rail as well.



To summarize my modifications:
1) I modified the 1117 "Battery" voltage regulator circuit to deliver 3.5V
(instead of the 3.8V) to get some voltage margin (dropout voltage 1.25V!)
and avoid issues there. I also added input (10uF) and output (100uF) caps.
]

To adjust the voltage, I put a 4k7 resistor in parallel to the existing 680E
(going to ground). Yeah, ugly...


2) Add a handful of decoupling caps to the VDD4P2 rail
10nF // 100nF // 1uF // 47uF
The VDD4P2 is on the cathode side of the fat shottky diode on the back side of the board.

I scraped away some solder-mask on a ground-plane and botched in a handful of caps.



Fragment of the reference schematics:


I'm quite sure that modification 2 alone will fix this issue, but I didn't bother to undo my other modification.

I hope this will help some Rigol DP832 owners to get their device working properly again!

/Thomas

Update: Running for one week now, without issues.

_Wim_:

--- Quote from: T2 on March 05, 2020, 06:26:56 pm ---Hi there,

I think I finally fixed it. My DP832 is now running without reboot for more than 24h.

--- End quote ---

Great work! I do not experience the reboots (yet?) of both of my DP832s, but it is good to know there is a solution when it happens. How old is your DP832? I wonder if this problem is related to a certain batch of instruments...

Smokey:
Great job! 

Who is going to be the brave soul that tries to simulate the conditions on a perfectly functional supply to see if it starts rebooting?

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