In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Beau Schwabe wrote:

[3]william chan,<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>[/3]

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[3]The 9.7V zener (D1) in your circuit is causing Q3 and Q5 to stay "on", defeating the purpose of the snubber circuit.<SPAN style="mso-spacerun: yes">  This circuit is designed to turn "on" in the event of an inrush positive voltage spike.<SPAN style="mso-spacerun: yes">  A reverse biased diode across the C-E junction of Q3 is all that is necessary to snub an inrush negative voltage spike.<o:p></o:p>[/3]

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[3]For diode capacitor filtering around the regulator, take a look at this link...<o:p></o:p>[/3]

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[3][url=http://forums.parallax.com/forums/attach.aspx?a=4331]http://forums.parallax.com/forums/attach.aspx?a=4331[/url]<o:p></o:p>[/3]

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[3]...Description of diode-capacitor filter:<o:p></o:p>[/3]

[3]Going from left to right, the first diode and capacitor combination off of the INPUT terminal to the regulator form a first stage "diode/capacitor filter". The second diode connected to the regulators GND terminal shifts the Ground reference by 1 diode (.6V) in preparation for the third diode. The third diode connected to the regulators OUTPUT forms a second stage "diode/capacitor filter". Since the third diode creates a diode voltage drop of .6V it is necessary to compensate that with the use of the second diode to shift the voltage up by .6V so that the net output remains at 5V.<o:p></o:p>[/3]

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[3]As far as using a pull-up resistor on the /MCLR, Chris is right.<SPAN style="mso-spacerun: yes">  The pull-up provides a slight amount of delay necessary for proper power up of the Chip.<SPAN style="mso-spacerun: yes">  This phenomenon is not isolated to the SX, many other processors with a comparable /MCLR pin will show similar behavior.[/3]

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