In a PIC controlled circuit, I would like to provide an auto reset
mode for the PIC during an undervoltage power supply situation, in order to
provide for a fail safe mode. The data books provide such a brownout
protection circuit. They also provide a circuit that compensates
for slow ramp up power supplies. My confusion is, how does one know if
their supply has a slow ramp up. There is no circuit listed that provides
slow ramp up compensation and brownout protection. Also, I know I have
seen inexpensive simple ICs that automatically activate an external
reset with a brownout condition, anyone know offhand of a chip #.
Seeking career in Instrumentation and Digital System Design
San Jose, California August 1996
>reset with a brownout condition, anyone know offhand of a chip #.
National Semiconductor LP2951 takes 0-30VDC input, regulates it to 5VDC @
100ma with a reset for your micro when 5V goes out of a 5% window.
8-pin DIP; less than a buck in quantity.
On your crystal question, with a conventional crystal or a ceramic
resonator, I typically use Zero ohms for series resistors. OTOH, I ALWAYS
use a 1 meg in parallel with the crystal or resonator for reliable startup.
If you're using a LF timing crystal like 32KHz, though, that's an entirely
different kind of animal.
"Ignorance is a renewable resource" P.J. O'Rourke
For undervoltage reset, reset delay timer, watchdog timer, and battery backup
you can't beat the Maxxim MAX693, MAX800 and company.
For undervoltage reset with a delay timer and a watchdog (no battery backup),
the Dallas or Linear Tech DS1232 are tops. I have 68000 based stuff with the
DS1232. You can drag a screwdriver across the bottom of the PCB and it will
come right back up.
For undervoltage only, no watchdog, I like the Dallas DS1233 family in a TO-92
or SOT. They come in 3.3 and 5V. Motorola makes one, too, I think it is
MC34??? but I really don't remember. The Motorola is a simple comparator. It
doesn't have the time delay that the others have.
You might think an external watchdog has no value for the PIC since it has one
internally. Not so! The PIC watchdog is really only good for trapping errant
firmware or soft errors in the PC, etc. Also, I have had trouble with the PIC
locking up, including the watchdog, on brownouts. An external watchdog can cure
this ruggedly. You can even switch the power of the PIC with the watchdog and
recover from latch-up but that's getting carried away.