AnsweredHot!Recommended dsPIC schematic design

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danbakana
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2018/06/09 15:54:57 (permalink)
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Recommended dsPIC schematic design

I'm having some problems with my dsPIC33EP64MC202. I believe it is a hardware problem, but I'm not sure yet.
I designed the hardware, designed the firmware, and everything was fine in debug mode. So, I stopped to use it in debug mode and I programmed it definitely. One month later it stopped with no reason, so I tried to debug it, but when I tried to load the code the a message appears to inform "Invalid ID" . I tried everything and well, it means the dsPIC died, am I right ? So I put another dsPIC, same model and it worked fine, but, aproximately one mounth later, the same thing happened, it stopped and when I tried to debug, the message appears: "Target Device ID (0x1d390000) is an Invalid Device ID"
Whell, I will put here my schematic and the specification in datasheet . I can see that the connection in vcap is not the recommended, but, if I do not put vcap in VDD, debug does not work. What can I do ? Would it be the responsible for the dsPIC fail ? The figure with my schematic and the recommended design is attached to this post under the name schematicDSPIC
 
I'm thinking it woul be because of the soure power, I use a LM317 to convert 13.8V to 3.3V. The device need to be functioning 24hs, is this source power a good choice ? The figure with my design of the source is attached to this post under the name fonte3.3V.
 
Any help would be great.
With my best regrads.
Daniel.

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#1
Bob White
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Re: Recommended dsPIC schematic design 2018/06/09 16:56:46 (permalink)
4.67 (3)
 
Is your schematic for the LM317 circuit correct? 
 
How did your calculate that this circuit would have an output voltage of 3.3 V?
 
Have you measured the output voltage of the LM317 circuit?
 
Also, where did you get a 375 ohm resistor? That is not a standard value (374 ohm is).
 
I calculate the output voltage as 1.25 V * [1 + (187.5 ohm/240 ohm)] + 50 µA * 187.5 ohm = 2.35 V
 
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davekw7x
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Re: Recommended dsPIC schematic design 2018/06/09 17:31:14 (permalink)
4.33 (3)
danbakana
...hardware problem...



Do you really have the MPU's Vcap pin connected to the Vdd power rail as your schematic shows?  That's a big no-no.  (Section 2.3 of the Data Sheet, DS70000657H)
 
If you can't debug without dinking with the voltage applied to Vcap, then I'm thinking that Vcap is inadequate.  (Must have low ESR; I use 10 uF, 16V MLCC.)
[Edit]
If Vcc is in spec and it isn't operating properly (producition or debug), maybe temporarily bridge extra capacitor(s) across C5.  This gives more capacitance and lower ESR.  I once did this for a client's board that had a Tantalum of unknown provenance.
[/Edit]
 
Regards,
 
Dave
post edited by davekw7x - 2018/06/09 18:16:47

Sometimes I just can't help myself...
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NorthGuy
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Re: Recommended dsPIC schematic design 2018/06/09 21:00:22 (permalink)
4.75 (4)
VCAP is the output of internal voltage regulator. It is less than VDD - some PICs have 2.5V (yours is likely one of them), others 1.8V. If you zap it with higher voltage (VDD in this case), it will eventually fry the PIC.
 
If VCAP pin doesn't produce the required voltage by itself, you have a problem with VDD or with capacitor.
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du00000001
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Re: Recommended dsPIC schematic design 2018/06/10 01:38:13 (permalink) ☼ Best Answerby danbakana 2018/06/13 04:39:35
4.75 (4)
Funny thing that the EP survives that long.
To summarize the colleagues' findings:
  1. The output voltage of the LM317 is too low. (While the LM317 in itself is a reasonable choice.)
  2. The too-low voltage (obviously gone undetected until now) didn't give the on-chip regulator enough headroom to create a stable voltage on Vcap.
  3. Your "remedy" to connect Vcap to your "3.3 V" supply (obviously not high enough to kill the EP instantly) is the reason for the repeated failures: you're frying the chip over time.
Remedy:
  1. Remove the connection between Vcap and your 3.3 V supply.
  2. Measure your 3.3 V supply and make it really 3.3 V (or something in the range 3.0 .. 3.6 V): remove R6 - R8. If I did not miscalculate, 37x Ohms for R6 and 240 Ohms for R7 (leave R8 unpopulated) should result in a nice 3.2 V - at least good to start with.
  3. Now you are free to replace the EP once again. Wish it a long life...

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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danbakana
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Re: Recommended dsPIC schematic design 2018/06/11 10:27:36 (permalink)
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Bob White, I'm sorry, it was my mistake, the schematic of the regulator voltage was not updated. There are no resistors of 375 ohms, but one of 412 ohms instead. So, the calculation will be:
 
1.25 V * [1 + (412 ohm/240 ohm)] + 50 µA * 187.5 ohm = 3,41V
 
,
 
 
 
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danbakana
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Re: Recommended dsPIC schematic design 2018/06/11 10:33:56 (permalink)
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Thank you very much for your reply. I'm sorry, it was my mistake, the schematic of the regulator voltage was not updated. There are no resistors of 375 ohms, but one of 412 ohms instead. So, the calculation will be:
 1.25 V * [1 + (412 ohm/240 ohm)] + 50 µA * 187.5 ohm = 3,41V.
Now, I will do the recommended modifications, but, there is no recomendation of C capacitor value at MCLR pin, I read some people talking about 100nF, but it was not official, do you have any tip ? Again, thank you for your help.
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du00000001
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Re: Recommended dsPIC schematic design 2018/06/11 10:51:21 (permalink)
4.5 (2)
The capacitance on /MCLR depends on your environment: if it is "clean", you could even go without.
If it is quite noisy, you may need some capacitance. The problem with that is that - depending on the value of the resistor(s) - you might have to add a diode as well to make sure the capacitor discharges fast enough in case of power failures.

PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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