Somehow I managed to internally short Vss and Vdd pins on Prop
AndrejaKo
Posts: 24
Well I finally killed my first propeller and I'm trying to find if my theory on how it died is correct.
It seems that I forgot to power up the right-hand side power rail to which the DIP Propeller and the the 24LC256 were connected. It appears that the 24LC256 was pulling power from the Propeller power pins and I suspect that this is the reason why the Propeller died. Does it sound plausible?
I didn't notice the problem until I noticed that the Propeller is hot and it seems that in described setup it worked fine for several minutes since I checked the voltages at both sets of power pins and they were correct 3.3 V
Just to be thorough: the left power pins were connected to a LF33V regulator which was connected to a 7805 which was connected to a 7812 which was connected to a 12 V 10 A unregulated power supply with actual output voltage of 19 V. The power supply has 30 mF worth of capacitors at the output and I had an additional 2.2 mF capacitor at the breadboard. The regulators all had 1 μF input capacitors and 100 nF output capacitors and I had 100 nF decoupling capacitors at the Propeller power pins and at the 24LC254 power pins.
It seems that I forgot to power up the right-hand side power rail to which the DIP Propeller and the the 24LC256 were connected. It appears that the 24LC256 was pulling power from the Propeller power pins and I suspect that this is the reason why the Propeller died. Does it sound plausible?
I didn't notice the problem until I noticed that the Propeller is hot and it seems that in described setup it worked fine for several minutes since I checked the voltages at both sets of power pins and they were correct 3.3 V
Just to be thorough: the left power pins were connected to a LF33V regulator which was connected to a 7805 which was connected to a 7812 which was connected to a 12 V 10 A unregulated power supply with actual output voltage of 19 V. The power supply has 30 mF worth of capacitors at the output and I had an additional 2.2 mF capacitor at the breadboard. The regulators all had 1 μF input capacitors and 100 nF output capacitors and I had 100 nF decoupling capacitors at the Propeller power pins and at the 24LC254 power pins.
Comments
What I'm asking is: Would pulling 400 μA from one side of the power pins be enough to cause an internal short between the Vdd and Vss on the Propeller?
There are a few ways of doing this... The usual is hooking up 5V supply to the Prop somewhere.
It's hard to imagine that hooking up an EEPROM chip the wrong way would do that.
I've had dead shorts on some Prop pins and had it survive.
Maybe not hooking up all the Vdd and Vss pins would do this, but I haven't seen that.
Putting 3.3V on Vss or Gnd on Vdd might heat of the Prop in a recoverable way, but I haven't tried that yet.
No, 400uA flowing the right way (sourced from a IO) will not harm anything.
With multiple supplies, there is another mechanism you need to watch for, which is Latch-Up.
On some data sheets you will see this given as a Current spec.
Most CMOS devices parasitic clamp diodes, are actually lateral transistors - so the PN clamp to Vcc, is the E-B junction of a PNP, with the
collector laterally located. Similar for the Vss clamp.
These lateral current flows mean you have a low gain SCR built in, just waiting to be triggered !!.
Trigger is usually via excessive clamp diode current, and good devices need > 200mA to fire this mechanism.
Sometimes ESD events can cause this, which can include long cables with significant floating capacitance being plugged in while powered.
TV monitors are a good example - if double insulated, they have no inbuilt capacitive discharge paths.
Once the parasitic SCR fires, it crowbars the power supply to a SCR drop, and depending on the powers involved, can be fatal. (classic HOT device)
The holding current of such SCRs can be surprisingly good - tests I did some years back on a Low-current limited supply, found under 10mA was needed before the SCR released on the CMOS devices I tested, and some needed almost 1A to trigger.
The Prop data sheet does not seem to spec this ? - but it is easy to test, the current pulse width can be << 1ms.
Is you have multiple rails, and power the Prop _last_, the other rail can supply current, or plugging in physically large loads etc.
So hooking up only one Vdd to the power supply and then the other to the EEPROM would cause current to flow across the two Vdd pins.
Though I admit, I don't think this explains the heat.
The resistance between any of the Vdd/Vss pins is around 9 Ω, so there was definitely some current going on.
As for the latchup part, if I understand the explanation correctly, I'd need to have current going through one of the non-power pins, right? When this happened, I only had the Propeller connected to the EEPROM.
I'll post the exact diagram in the morning, since it's 0:41 here right now.
Usually it is tested as current injection into IO pins, tho you can also trigger this with a supply voltage spike.
Were you connecting/disconnecting anything, or powering any other equipment on/off ?
I also use a bench power supply with adjustable current limit and set the limit low on first test, ready to kill the power if too much current seems to flow.
And for this incident I'd tend to agree that SCR latchup is likely - the chip getting hot is a classic sign. I've had an ADC chip do this at swichon for some mysterious region (probably spike on an input), but it survived after power cycling. Protection against input over- and under-voltage can be done with schottky diodes in parallel with the on-chip protection diodes (schottky diodes have lower forward voltage and prevent the on-chip diodes conducting)