The reason for the capacitor on the prop-plug dtr npn
TransistorToaster
Posts: 149
Hello,
On the prop plug:
www.parallax.com/Store/Microcontrollers/BASICStampModules/tabid/134/List/1/ProductID/398/Default.aspx?txtSearch=propeller+plug&SortField=ProductName%2cProductName
there's a DC blocking 10nF capacitor between the FTDI USB to serial converter DTR and the NPN base. I was just wondering why this particular solution was chosen?? Clearly, now we have to flash the DTR line to reset the propeller for programming, equivalently, it is not possible to hold the propeller in the RESn=low.
Could this circuit been done as such to prevent noise from causing jitter on the RESn?? The NPN is performing as a high gain common emitter inverting amplifier with the Vbe is essentially 0V, thus it should be essentially off. Having a capacitor or not would not make a difference under this perspective.
Thanks,
Frank
On the prop plug:
www.parallax.com/Store/Microcontrollers/BASICStampModules/tabid/134/List/1/ProductID/398/Default.aspx?txtSearch=propeller+plug&SortField=ProductName%2cProductName
there's a DC blocking 10nF capacitor between the FTDI USB to serial converter DTR and the NPN base. I was just wondering why this particular solution was chosen?? Clearly, now we have to flash the DTR line to reset the propeller for programming, equivalently, it is not possible to hold the propeller in the RESn=low.
Could this circuit been done as such to prevent noise from causing jitter on the RESn?? The NPN is performing as a high gain common emitter inverting amplifier with the Vbe is essentially 0V, thus it should be essentially off. Having a capacitor or not would not make a difference under this perspective.
Thanks,
Frank
Comments
It's much simpler than that. You don't want the DTR line to hold the Propeller in reset when the serial port is not open. You would like to reset the Propeller when the port is first opened and DTR goes from false to true. A capacitor is a cheap, physically small way of accomplishing this. Depending on how the other circuitry is done, it might also reset the Propeller when DTR goes from true to false. That's tolerable.