PIC Programmer II
Posted on Oct 6, 2012 11750
C1 is a charge pump capacitor*. This works voltage doubler. When TxD is negative voltage, C1 is charged through D7 from GND. If TxD carries out turn-on, since it will become positive voltage, the voltage charged in C1 is raised. The created high voltage is regulated by D7 to about 13V. If TxD carries out turn-on, both supply voltage will be created. CTS and RTS also join creation of supply voltage. This VDD is delayed and is applied by C2 and R4. D1-D5 are clump diodes. Since voltage drop is within the limits which can be disregarded, general purpose small rectifier is sufficient as the diode to be used. e.g. 1N4148,1S1588(Toshiba),1S2076A(Renesas or Hitachi),1S133(ROHM) . When not succeeding by the device which needs programming current like 16F84(A), the value of R3 is made small. Probably, it will be good using a trimmer resistor as R3.
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After I published the RCD Programmer on my Web site, it has passed in one year. I obtained various questions about the RCD Programmer from many people. And Microchip newly developed Low-Pincount PICs which built in the memory exceeding 1k words. I recommend change capacity of C1 to 470uF now for stable programming. When you insert a RCD Programmer into the COM port of your computer during operation("Hot Plug-in" or "Hot Socketing"), inrush current is generated. It seems that it is weak to an overcurrent and a short circuit although "EIA/TIA -562" is power saving and compatible with "EIA/TIA-232-E." So, I added 200-ohm current limitation resistor. This resistor limits peak current and help computers which have poor surge protection circuit inside. However, the RCD programmer will not work well as for such a computer because the voltage of the COM port might be very low. *About general description of charge pump, please refer to DC/DC Conversion without Inductors(Maxim/Dallas Application Note 725). A important point is not a charge pump but controll of VDD in the RCD Programmer circuit. If this point is excluded, the JDM Programmer is far more excellent than the RCD Programmer. Because the RCD Programmer works by the pulse only once, the capacitor power cannot be continued for a long time.