Post by thread:brownout detection (BOD)

This is half question/half cautionary tale. When I set up my MPLAB project for the 16F628, either I chose to turn on brownout detection (BOD) or it defaulted to on. It's battery powered, using 3x AA for a net VDD of about 4.5V. When I switched from using one surplus 7-segment display at low power (for testing) to my new MAN8610 (very nice .8" digit displays) at full power with multiple segments, I could not get the PIC to stay on. In addition to the normal decoupling (.1u and 1.5u) that I had been using all along, I needed almost 100uF to smooth the power out enough to drive the displays and keep the PIC on. When I finally realized I was hitting BOD, I turned it off, and it works without the 100uF. So my question is: Is this an acceptable solution, or is the fact that I *would* be triggering the BOD a sign that I can expect problems anyway? -- Ben Jackson <spam_OUTbenTakeThisOuTben.com> http://www.ben.com/ -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Ben Jackson wrote: > This is half question/half cautionary tale. When I set up my MPLAB > project for the 16F628, either I chose to turn on brownout detection > (BOD) or it defaulted to on. It's battery powered, using 3x AA for > a net VDD of about 4.5V. > > When I switched from using one surplus 7-segment display at low power > (for testing) to my new MAN8610 (very nice .8" digit displays) at full > power with multiple segments, I could not get the PIC to stay on. In > addition to the normal decoupling (.1u and 1.5u) that I had been using > all along, I needed almost 100uF to smooth the power out enough to > drive the displays and keep the PIC on. > > When I finally realized I was hitting BOD, I turned it off, and it > works without the 100uF. So my question is: Is this an acceptable > solution, or is the fact that I *would* be triggering the BOD a sign > that I can expect problems anyway? Ben, the problem is related to the 3xAA cells (probably carbon cells, right?). They have a not so low internal impedance, and the peak currents when multiplexing segments was high enough to drop the voltage of those cells to a BOD point. The 200uF capacitor in parallel with the cells accumulated enough energy to supply it fast to the multiplex current pig. As the capacitor has an impedance much lower than any regular carbon cell, the capacitor was the power supply for the multiplex and kept the cells with a voltage more stable. If you think about it, your circuit probably have some small caps around. The LED multiplex was trying to suck current from the AA cells and it "was not there" (due the cells impedance), so, the circuit ended up sucking current from the small caps you have in the board, and this can cause all sort of problems. A larger cap in parallel with the cells is a solution. Some other 47 to 100uF cap in parallel to the (anode) + side of the mux leds can also help the mux to have energy very close, not needing to cause any kind of noise or disorder in the current. If later you will use a regular power supply, it will have some sort of voltage filter, as a capacitor or a voltage regulator that will take care of this issue. If instead of carbon AA you used NiCad or NiMH cells, even with a lower voltage (3.6V for 3 cells or 4.8V for 4) it would never happens, since those cells have a very low internal impedance and would feed the led mux without any significative voltage dropout. You need to do some lab tests, your cells versus power drain. Measure everything individually, what consumes how much current, and draw a table about it. You can then try to optimize power consume to meet your wishes. If you intend to keep using carbon cells, you probably will need to substitute some power pig units for more skinny ones. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> This is half question/half cautionary tale. When I set up my MPLAB > project for the 16F628, either I chose to turn on brownout detection > (BOD) or it defaulted to on. It's battery powered, using 3x AA for > a net VDD of about 4.5V. > : > When I finally realized I was hitting BOD, I turned it off, and it > works without the 100uF. So my question is: Is this an acceptable > solution, or is the fact that I *would* be triggering the BOD a sign > that I can expect problems anyway? The way I read the data sheet, the PIC should operate as low as 3.0V (2V for L version), while the brownout detect is nominally at 4V (and MAY be as high as 4.4V), so the occurance of a Brown-out reset doesn't necessarilly mean that you are operating the PIC in an unsafe region. It looks like the BOR is designed for system with "real" 5V power, so essentially if you're operating at less thn 5V nominal, you probably shouldn't used the BOR feature. National Semiconductor goes so far as to offer different CPU versions for different power supplies, with different BOR criteria to solve this sort of issue. I haven't formed an opinion as to whether that helps much or not. Seems to me you can still manage to confuse yourself. BillW -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics