LED state meanings


8 posts
by 10-10-20 » Tue Jun 05, 2012 8:52 am
Hi, I'm troubleshooting a boot problem with my Pi from RS.
I seem to need to leave the Pi to 'warm up' for a number of minutes before it boots, but I'm getting three different LED states and I'd like to know what they mean as I can't see them documented.

1. Quickly flashing PWR LED (3/4 times per sec). I get this with the SD card removed.
2. Slower flashing PWR LED (<2 times per sec). I get this with the SD card inserted, but the system does not boot.
3. Solid PWR LED. I get this when the system eventually boots after 5/6 minutes.

What does the 'slower flashing' state mean?

Plus, who decided to make the PWR LED red..? It looks like an error-state notification.
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by MrBunsy » Tue Jun 05, 2012 8:56 am
My understanding was that the power led is hard-wired to the power supply, suggesting you've got some serious problems somewhere! Try a different power supply?
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by 10-10-20 » Tue Jun 05, 2012 11:57 am
Hi, the PSU came from RS and seems fine.
It reads 5.01v between TP1 and TP2.

The PWR LED doesn't appear to be hard-wired - the flashing is too regular to be the PSU, so must be GPU generated.

I have tested with multiple SD cards and what I have found is that once the system is 'cold', none boot, but once it's warmed up, all except one SD card will boot. So I'm suspecting a cold solder joint.

I'm going to perform a hair-dryer test on it tonight.
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by MrBunsy » Tue Jun 05, 2012 12:08 pm
I had a little looksee, the schematics ( http://www.raspberrypi.org/wp-content/u ... s-R1.0.pdf ) do show it as hard wired to the 3.3v rail, which comes straight from a regulator. I assume this schematic is still correct for the production boards?

Have a check of the 3.3v rail?
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by 10-10-20 » Tue Jun 05, 2012 12:22 pm
Ah ha! I couldn't find the schematic when I looked. Thanks for that.
So, if that schema is correct, my 3.3v bus is bouncing. I'll put it on the oscillicope on it and see whether it's doing so and report back tomorrow.
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by 10-10-20 » Tue Jun 05, 2012 5:47 pm
OK, good news. I've found the problem and it's not with the Pi, it's the Radionics PSU.
While the Pi was flashing it's PWR LED, I measured the voltage between TP2 and pin-3 of the 3.3v regulator RG2. It was reading 3.03v, then dropped to 2.90v then rose to 3.27v and then dropped again in the space of ~ 10 seconds.
I then retested TP1 and TP2 and found that they were also reading between 2.7v to 3.3v. So, I found another USB PSU and powered it up from that, and hey-presto, it booted straight up.

So the RS PSU is unstable. It was bought with the Pi.

So - if you're getting a flashing red PWR LED, it's likely to be the PSU delivering bad voltage.

PSU: RS: 7263069, 5.0v @1200mA.
Model: HNP06UK-MicroUSB.
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by MrBunsy » Tue Jun 05, 2012 10:20 pm
That is rather surprising! The PSU I got from RS works like a charm, but at least you've got it working now. Worth getting it refunded?
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by jbeale » Wed Jun 06, 2012 1:10 am
MrBunsy wrote: I assume this schematic is still correct for the production boards?

The schematic is mostly correct for the current Model B production boards. Here are the known differences: http://elinux.org/RPi_schematic_errata#Verified_typos
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