Brown-out reset on LF PIC

[bananasiong]

Hi,
I just wondered, if I use a LF PIC with less than 5 V, let's say 3.5 V for example. If the brown out reset is enabled, will the PIC always be reset? I have read the datasheet, the typical brown out reset voltage is 4 V, 3.65 V min and 4.35 V max. Should brown out reset be disabled if the PIC is running between 2 to 4 V?

Thanks

 

[justDIY]

which PIC? some of them are adjustable on the brown out reset.

unless you've got a real need for BoR / BoD, disable them, since it can cause the pic to act strangely if you're not prepared

 

[bananasiong]

which PIC? some of them are adjustable on the brown out reset.

unless you've got a real need for BoR / BoD, disable them, since it can cause the pic to act strangely if you're not prepared

I didn't know that the brown out reset voltage can be adjusted . I'm referring to the PIC16LF87XA type.
Any brown out reset voltage of the PIC can be adjusted?

 

[justDIY]

microchip's website comes up "no results" for 16lf876a and lf877a ... however there is a 16f87 which runs at 2.0v (low voltage).

I think the BoR circuit should work fine, since it is referenced off Vdd. If Vdd = 5v, Vbor = 4v, if Vdd = 3v, Vbor = 2v

Some of the "new" 16F and many of the 18F (or 18LF) chips have programmable reset and brown-out detect voltages, based on an internal reference. If battery power is a critical part of your app, you may want to consider a pic with those options.

 

[Gayan Soyza]

Most of the time I’m turning off my BODEN fuse settings. Only PWRT enable.

When the AC power is not available the circuit runs in battery with no problems.

 

[bananasiong]

microchip's website comes up "no results" for 16lf876a and lf877a ... however there is a 16f87 which runs at 2.0v (low voltage).

No, there is no result for all the PIC16 MCU. But you can see wheher there is LF types on the odering page. And now I have a few PIC16LF873A, it is impossible if it is not available.

I think the BoR circuit should work fine, since it is referenced off Vdd. If Vdd = 5v, Vbor = 4v, if Vdd = 3v, Vbor = 2v

Are you sure with the statement mentioned above? Or just a guess?

Some of the "new" 16F and many of the 18F (or 18LF) chips have programmable reset and brown-out detect voltages, based on an internal reference. If battery power is a critical part of your app, you may want to consider a pic with those options.

Yea, I'm looking for these PICs.

BTW, what are the PIC which are categorized under nano watt technology? Are they LF version of PICs?

Thanks

 

[geko]

I think the BoR circuit should work fine, since it is referenced off Vdd. If Vdd = 5v, Vbor = 4v, if Vdd = 3v, Vbor = 2v

On the 16F62xA devices the Brown out reset voltage Vbor appears to be fixed and not referenced from Vdd (see parameter D005 in datasheet). The figures shown would indicate that with a supply volatge of 3.5v the device would be held in reset.

 

[justDIY]

i guess there's a concrete way to find out ... hook the processor up to a battery and see what happens

I use a 12f683 with brown out reset in a battery powered app, and the chip runs routinely below 3v. When the bats are fresh, I maybe see 3.2v, and before long they are down to 2.5v. The circuit actually works well to just under 2.0v, but once the voltage goes below 1.9v the pic starts resetting (BOD kicks in). By checking the flags to find what the reset cause was, if I find it was a BOR, the program will sit in a 'standby' mode and refuse to turn on the rest of the circuit (which is a switch mode boost converter powering ~25v of leds)

 

[bananasiong]

i guess there's a concrete way to find out ... hook the processor up to a battery and see what happens

I use a 12f683 with brown out reset in a battery powered app, and the chip runs routinely below 3v. When the bats are fresh, I maybe see 3.2v, and before long they are down to 2.5v. The circuit actually works well to just under 2.0v, but once the voltage goes below 1.9v the pic starts resetting (BOD kicks in). By checking the flags to find what the reset cause was, if I find it was a BOR, the program will sit in a 'standby' mode and refuse to turn on the rest of the circuit (which is a switch mode boost converter powering ~25v of leds)

How is the processor going to know that how much is the supply voltage and hence, the brown out reset voltage is Vdd-1V?
I think for those PIC which don't have program selectable brown out voltage and the LF version is available, the BOR should be disabled if the supply voltage of lower than 4 V is used.
I just read through the datasheet for PIC16F883 and found that the brown out reset voltage is selectable 2.1 V and 4 V.

 

[Papabravo]

How is the processor going to know that how much is the supply voltage and hence, the brown out reset voltage is Vdd-1V?

A ratiometric BOD can be made with a resistive divider. For a fixed BOD any convenient junction can be used to provide a reference. My 12F683 datasheet lists Vbor as 2.0V Min and 2.2V Max. When no typical value is listed you can assume the midpoint, but you could be wrong in the case of an asymmetric or other non-normal distribution.

I must confess however that it seems strange for Vbor to be at say 2.2V when Vdd has a listed minimum 2.0V -- Go figure.

 

[bananasiong]

I must confess however that it seems strange for Vbor to be at say 2.2V when Vdd has a listed minimum 2.0V -- Go figure.

That's right.
I just realized that there is no LF version for PIC1688X, but they are working as low as 2 V when the operating frequency is less than 8 MHz. This is better as people don't need to buy two different type of the same PIC.