Problem when PIC power on!!!

[tienchuan]

Hi all.
I've a problem when PIC power on. In my circuit, i've used large capacitor (0.022F-5.5V) to built in power backup for PIC, for using save data in eeprom when lost main power.
Here's schematic of circuit:
https://docs.google.com/file/d/0Bz215_lkn4g-cXlvSHFjX2RwVnM/edit
I used to Brownout Reset function at 2.8V to reset PIC, but when I power on, PIC is not run.
I used PIC18F4680, CCS C complier ver 5.021.
My config fuses code:

[dyeatman]

Please show your crystal frequency line and tell us what crystal part
number, frequency and value of caps you are using.

Are you running the PIC at 5V?
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[tienchuan]

Thanks u.
I'm running PIC at 4.4V ( VCC-> diode-> VCC-PIC), VCC:5.2V from adapter.
I'm using crystal 10MHZ, 2pin, dip type and no part, 2 caps 22 p.
My schematic of PIC:
https://drive.google.com/file/d/0Bz215_lkn4g-NGNNSlhVZVpNUmM/edit?usp=sharing
And my code config:

[dyeatman]

Crystal hookup looks ok on the surface.

You are running awful close to minimum (4.2). Your meter error may be
that much. What happens if you apply a full 5V?

Also try using HS instead of H4 and set the use_delay to 10Mhz to see what
happens. It may be the PLL wont fire up at that low a voltage.
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[tienchuan]

I've tried with setup oscillator 10MHZ with HS fuses, but it still don't work:|
Have another problems ?
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[dyeatman]

Try HS/10Mhz at 5V with just these fuses
INTRC, NOLVP, NOWDT, MCLR

I left out PUT because I have seen it cause startup issues.

If this doesn't work I suspect something other than the program.
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[PCM programmer]

Read Ttelmah's comments in these threads about saving eeprom
during power-down of the PIC:
http://www.ccsinfo.com/forum/viewtopic.php?t=44144
http://www.ccsinfo.com/forum/viewtopic.php?t=38936

[Ttelmah]

Key is that you need to build your own MCLR circuit to actually hold the PIC in reset until the supply has properly stabilised.

On some PIC data sheets you will find there is a maximum time specified for the supply to get to the working voltage, for the internal reset to work. All PIC's actually have this limit, but only some 'tell you' about this. If a supply rises too slowly, the POR is not generated, and often the oscillator hasn't started, and the kick needed to start this doesn't happen.
If you look (for instance) at the data sheet for the PIC18F4620, you have a section for 'power on reset', and the comment:
"External Power-on Reset circuit is required
only if the VDD power-up slope is too slow.
The diode D helps discharge the capacitor
quickly when VDD powers down."
They then specify that the supply must rise at a minimum of 0.05V/msec, for the internal MCLR to be used. So no more than 100mSec to 5v.

On many crystals/PIC's, adding a large resistor (perhaps 1MR) across the crystal, will ensure it starts. Then if you add an external MCLR circuit, to hold the MCLR low for slightly longer than the time needed for your supply to reach it's operating voltage (remember to have the diode across the pullup resistor on this to avoid damaging the PIC), then gets everything to work.

As a comment though, have you actually calculated how much capacitance you need to keep the PIC running to write the data (simple discharge calculation based upon the current drawn by the PIC)?. Generally a capacitor of only a few uF is enough to power a PIC to write a dozen or more bytes to EEPROM. You may be creating your own problem by going rather too far.

Generally PUT, should always be used with a crystal, and shouldn't cause any problems. It'll only cause problems where your external circuitry requires lines to be operated as soon as the PIC boots, and in these cases PUT should be removed, but then your code should, before anything involving 'timing' is done, use the status bit to check that the oscillator is stable before proceeding. Otherwise quite a few things will 'time' incorrectly for the first few mSec after boot. That is what PUT is there to stop. It is not normally wanted or needed with INTRC.

As a further comment, you say that you are using HS, and 10MHz, then show fuses for INTRC (which can't do 10MHz).

I see PCM_programmer has posted pointing to some earlier threads about power up problems, while I was typing).

[ckielstra]

[asmboy]

[tienchuan]

Thanks you Ttelmah.
A few day, I tested again board but result not achieved, the circuit sometimes run corect, not stop when power star.
I think I have a problem with large capacitor and algorthim to save data when power loss.
I searched on forum and saw your topic discussion about how choosing buffer capacitor. And I saw this equation to calculates the value of buffer capacitor.
C= I*t/(delta)V
In my case, when power loss, the HLVD interrupt is trigged, and it perform task read 2 byte address, and write 6 byte data, final, write 2 byte new adress again. So that, i need 5*10=50ms to save data in external EEPROM, ensure i choose time equal 60ms, I caculate that I needed 1000uF buffer capacitor (i=10mA, t= 60ms, delta V = 4.62-4=0.6V).
Follow your idead,I think before I choose large capacitor because i dont using right osscilator when power loss, i need to switch osscilator to minimum to save more power for writing in eeprom. Can you show me find a way to switch and choose right osscilator?
About the reset circuit, I only use resistor pull up because i read a manual of pickit3, so i careless about it, i 'll checked it again.
Thanks
About the reset circuit, pr


I tried with your suggest, adding 1MR resistor parallel with crystal, but it
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[tienchuan]

[tienchuan]

[Ttelmah]

If your calculation says '1000uF', then why use 22000uF?.

The EEPROM takes 4mSec/byte, not 10mSec/byte, so your calculated value is already 2.5* larger than needed, you are running with at least 50* the capacitance needed....
Big problem is that the capacitor make take an age to fully discharge. Once the voltage on it drops below the point where the PIC oscillator stops, consumption plummets, so the chip may well be sitting with a couple of volts still on the capacitor, several minutes after you switch 'off'.

[tienchuan]