Question/Trouble in using #fuses

chen · Joined: 03 Mar 2004 Posts: 11

I am using 18F8620 in my design, and my application requires Brownout detect with Brown out voltage at 4.2V.

1) When I use #fuses to set up BROWNOUT as:

#include <18F8620.h>
#fuses HS,NOWDT,PUT, NOPROTECT, NOLVP, BROWNOUT

I notice the HEX file generated by CCS still has no BROWNOUT flag:

:020000040030CA
:0E00000000220C0E83030100FFC0FFE0FF4052

2) How do I specify Brown out voltage 4.2V?

3) When brownout is detected, will the PiC simply reset? If not, what will it be?

Thanks for your attention/time!

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

What compiler version are you using ? I compiled this test program
using your fuse settings with PCH vs. 3.188, and it worked OK.
See the results for Config word 2, below:

chen · Joined: 03 Mar 2004 Posts: 11

I am using Picc 3.186

BTW:

How do I specify 4.2V brownout voltage?

How do I detect a brownout from the software? (Checking RCON register?)

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

chen · Joined: 03 Mar 2004 Posts: 11

Thanks for your quick reply.

I tried BROWNOUT, and it simply doesn't work at all with CCS 3.186, but BORV42 works fine.

I will try some later version of CCS to see if it works there

chen · Joined: 03 Mar 2004 Posts: 11

I just tried CCS 2.212, and the fuses were set correctly.

Now, a new question:

When I gradually lower the power supply voltage from 5 all the way down, until the PIC quits, I don't see the BrownOut reset, which is set at 4.2V

I also tried to lower it to 3.5V then back to 4.5V, I still don't see brownout reset

How is it supposed to work?

Thanks again!

treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

don't know it your hardware is buit yet or if you have pins available,...but
you could read analog volt from A pin on PIC and tie a output pin with a pull-up res to your reset. I know its not pretty, but it may work for you.

AND you could do a little more processing before the power dies.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Please check the config bits by reading the PIC with your programmer.
Make sure that the correct bits are set:
BOREN = 1

// The following setting gives 4.2v
BORV0 = 1
BORV1 = 0

Also they recommend that the Power-up Timer should be enabled
when using Brownout reset.

\PWRTEN = 0 (ie., True)

See the reference manual:
http://ww1.microchip.com/downloads/en/DeviceDoc/39503.pdf

After you've checked all these things, if it still doesn't work, I will
drag my heavy variable power supply out of the closet and test BOR
with a 18F452 or 458. But I'm asking you to check everything first.

chen · Joined: 03 Mar 2004 Posts: 11

Thanks, I will confirm that.

According to the 18F8620 user manual (section 3.5) and my setup, brownout reset should occur when supply is below 4.2V for more than 200us, and the chip will remain in brownout reset until Vdd rises above BVdd.

Since my Vdd drops slowly, once it is below BVdd, it will never come back above it, so it is not really a brown out here.

I wonder if there is a way for me to get a warning of Vdd is below 4.2V (without adding power monitoring circuit outside of PIC).

Anyway, I don't think it is CCS' problem, for the HEX file does have the correct flags set.

Thanks again for your time!

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I tested Brownout reset with the 18F458, and it works completely as
expected.

This is the test program I used:

chen · Joined: 03 Mar 2004 Posts: 11

PCM programmer & treitmey:

Thanks for your time

As I stated in my last post, I finally realized my test condition was not really a brown-out case, but a slow discharging, thus I will not see the PIC come out off the brown-out reset

I may have to use some external circuit to generate the interrupt so that I can do some final clean before the PIC dies.