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, for 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, 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. 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