Would you run a 16F688 at 95 mA?

[djulien]

The 16F688 data sheet lists 90 mA as the max for Port A + C*. I was originally going to run some LED strings at 20 mA and that meant moving to an alternate 16F part with a higher total port limit. However, I've found that the LED strings look bright enough at 15 mA. That means I could stay with the 16F688, but I would be running it at "the limit".

Would running it that way reduce its lifespan (more so if some of the resistors are a little off), or possibly make it more vulnerable to noise as it switches LEDs on/off?

* There was another thread on that topic:
http://doityourselfchristmas.com/forums/showthread.php?t=11320

thanks

don

[P. Short]

How are you running a 'string' on one pin of a PIC16F688? Unless you have a buffer of some sort (which probably renders the PIC16F688 current spec moot), you probably can only run one LED off each pin (subject to the 90 mA limit).

[djulien]

How are you running a 'string' on one pin of a PIC16F688? Unless you have a buffer of some sort (which probably renders the PIC16F688 current spec moot), you probably can only run one LED off each pin (subject to the 90 mA limit).

Yes, there is one LED anode in series with a resistor on each of 6 high I/O pins at a time so they will be sourcing 15 mA each (or maybe 20 mA if I move to another PIC), which would draw a total of 6*15=90 mA, and there is also a PNP transistor on one low I/O pin to serve as the buffer to drive the LEDs in a common cathode configuration. Attached is a partial schematic (this is part of a chipiplexing circuit; there are more LEDs in parallel on other I/O pins). So I am figuring that 6 I/O pins at 15 mA will give me the 90 mA max.

don

[P. Short]

The usual chipi-plexing circuit that I've seen on the web has the emitter and collector reversed from what you drew.

You might want to consider the PIC16F627A. It is only slightly more expensive than the PIC16F688, has more pins and a higher current rating (250 mA on VDD, 300 mA on VSS). It has less program space, and some open-drain silliness on the RA4 pin.

[P. Short]

The usual chipi-plexing circuit that I've seen on the web has the emitter and collector reversed from what you drew.

You might want to consider the PIC16F627A. It is only slightly more expensive than the PIC16F688, has more pins and a higher current rating (250 mA on VDD, 300 mA on VSS). It has less program space, and an open-drain quirk on the RA4 pin.

[djulien]

You might want to consider the PIC16F627A. It is only slightly more expensive than the PIC16F688, has more pins and a higher current rating (250 mA on VDD, 300 mA on VSS). It has less program space, and an open-drain quirk on the RA4 pin.

I really need the flash and RAM space, so I would need to use the '648A rather than the '627A. I'm not up to speed on the differences between EUSART and USART. The '689/690 sounded more like the '688 than the '627/'648; I'll need to think about this some more.

thanks!

don

[P. Short]

The PIC16F627A/etc has a pinout which is quite similar to the PIC18F1220 pinout. The slightly enhanced instruction set and register set of the latter part might be useful in some situations.

[djulien]

The PIC16F627A/etc has a pinout which is quite similar to the PIC18F1220 pinout. The slightly enhanced instruction set and register set of the latter part might be useful in some situations.

Okay, that might make a good reason to "move up" to 18F. Thanks for the suggestion.

don