Please explain why this kind of a connection is used ??

[arunb]

Hi,

I usually see this kind of a network in Reset/MCLR pins..

The reset/MCLR pin is connected to one end of a capacitor (usually 0.1 micro F) and a 10 K resistance. The other end of the cap is connected to +5 V and the other end of the resistance to GND.

Kindly explain why this kind of a network is used ??

Also is it OK if I connect the pin directly to +5 V, is the above network really essential ??

This is essential because I usually connect the reset./MCLR pins directly to +5 V in all my previous designs (and these have worked).

thanks
arunb

[davekelly]

It is most likely the resistor is connected to VCC, and the capacitor to GND.

It holds the device in reset for a few ms while the capacitor charges. It isn't usually necessary on most PICs, as they have a powerup timer fuse

Best practice is to still use a pullup resistor on the MCLR line, of nominally 10k

[Ttelmah]

Historically, the resistor/capacitor connection was needed on the older PIC's that didn't have a power up timer. Normally with the resistor also bypassed with a reverse diode to give rapid discharge of the capacitor, when the power is turned off. Using a resistor, in general when connecting pins, even when tying them directly to a logic rail, is quite a 'good thing', since it limits the maximum current that can flow if something goes wrong. With an input only pin, it shouldn't matter, but some CMOS inputs, can suffer from input latch-up, and if the resistor is not present, and this happens, it destroys the chip. The PIC is pretty well protected against this. A resistor is often still used, since this pin needs to be pulled up to Vpp, for ICSP.

Best Wishes