> When use the serial interface and differentiate the reset signal using a > series capacitor (like you did) understand that a +/-12V reset signal which > may come from the serial line (in fact is +/-15V if you read the EIA232 > standard or even +/-25V if you refer to older 232 standard) will create a > rising pulse of +12V and a falling pulse of -12V at the output of that > capacitor (the pin connected with MCLR). Since MCLR has fortunately the > negative clamping diode to GND inside the PIC, the negative pulse is limited > at -0.45...-0.6V, but the positive pulse above VDD (where VDD is the supply > voltage of PIC) still exist. So, it was just the hazard that you didn't > killed the PIC with this reset circuit, because the MCLR accept higher > voltages up to Vpp level. Newer PICs have Vpp lower and lower. So a good > reset will provide THE SAME logic level and THE SAME minimum reset pulse no > matter which value has the power supply or the voltage on the serial line > (which could be as low as +/-6V for a laptop, +/-10V for a desktop or +/-15V > on a serial terminal). > > Vasile
Thanks for your feedback on this, I haven't had time to reply. You must be referring to my schematic: http://www.borntechi.com/temp/ICSP_BOOTLOADER.JPG with RT reset. I see how the series cap on RT could give too high or too low of a voltage for MCLR if I get the signal direct from the serial port. However, I am using a MAX232 level converter. I put that capacitor there so I can get a spike down to 0v to get the reset (because RT is the inverse of what I want). It could still give me a positive spike up to +10v, but MCLR accepts voltages up to +13v anyways, so I guess I'm ok there. I don't think I will get any spike less then 0v since the normal voltage is 5v (I do get a spike down to 0v). I just made a new schematic: http://www.borntechi.com/temp/ICSP_BOOTLOADER2.JPG, This new schematic should have sufficient 1ms reset during power up via R6/C7 Does anyone see any dangerous issues with my new schematic? Thanks, Matt. -- You received this message because you are subscribed to the Google Groups "jallib" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/jallib?hl=en.
