On 4/24/07, Xiaofan Chen wrote: > On 4/24/07, Olin Lathrop wrote: > > Xiaofan Chen wrote: > > > I think a lot of the design has a 10nF (I've even seen 100nF) > > > capacitors on the MCLR/Vpp pin for EMC reasons. I think you should put > > > it there. > > > > But why? What purpose does it serve? A simple 20Kohm pullup to Vdd > > is just fine. > > > > Not really. A simple 20k Ohm pull-up is okay for Power-On Reset. > > However, MCLR/Vpp can be one of the weakest link for many PICs. > I've seen quite some more reliable design put an external IC, a pull-up, Sorry it should be an external *reset* IC. Normally the PICs are quite good and no external reset IC is necessary. > a 10nF capacitor to ground. Some of them also have a schottoky diode in > series with MCLR. > > In fact, if you read many PIC datasheets, the recommended connection > for MCLR often has a 10nF capacitor. For example, I just opened the datasheet > for 16F684 and it has the 10nF capacitor even though it says that this > 10nF capacitor is optional and not critical. However last time we encountered > EMC problem with a 16F819 without this 10nF capacitor. Actually the datasheet for 16F72 and 16F7x7 show 0.1uF capacitor. Therefore it is really a very common practise to put a bypass capacitor to Vpp. > As other people have mentioned as well, the capacitor is also good for > reliable power on reset in the case of slower Vdd power up. An example > is the datasheet for 18F2620. > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist