DL4YHF's PIC-Keyer

last updated: June 2002.

Power-Down mode of the PIC keyer

The keyer software turns the internal oscillator off to save power after a short time of "no activity". This state is called "power down"-mode. It is a special feature of PIC processor used in this circuit. You may find more info about this feature in the source file of the keyer software which is available on the author's homepage. With a PIC 16F84 running at about 2.5 Volts, the power consumption should be 1 microampere (maybe less) as long as the keyer is in "power down"-mode.

To achieve minimum power consumption during power down mode, some care has to be taken:

  • No input pin may be "floating". That means, the logic level on every pin must be either a good logic "low" or a good "high" level (0 V or 2.5 V but nothing "in between").
  • No input pin with internal pullup resistor may be connected to ground during power-down.
  • No output pin may source (or sink) DC current. Important, because output pins of the PIC remain active output pins also in "power down"-mode.

PORTB has its internal pullup resistors enabled, so there is no need to tie unused pins of port B to ground or VCC. It would increase the standby current if you pull any of these inputs (RB0...RB7) to ground !

PORTA has no internal pullup resistors. As most of the pins of port A are outputs, you don't have to worry what to do with these pins... just leave all outputs open if you don't need them (RA2, RA3).
The speaker output (RA3) should only drive capacitive loads like piezo speakers. In case of doubt, use a series capacitor of about 100nF to ensure that no DC current can flow across RA3 duing standby.
All inputs of port A (like RA4, which was once intended as a frequency counter input) may be tied to ground if they seem to be "floating".

If you still can not get the standby power consumption down to 1uA (at 2.4 Volts DC in), check the voltage of all inputs and outputs with a high-impedance DC voltmeter (10 MOhm or more) when the keyer is in standby mode. Here are the DC voltages measured against ground directly at the PIC pins:

    Pin1:   0.00V   Pin2:  0.00V    Pin3: 0.00V     Pin4: 2.49V     Pin5: 0.00V 
    Pin6:   2.49V   Pin7:  2.49V    Pin8: 0.00V     Pin9: 2.49V
    Pin10: 2.49V   Pin11: 2.49V   Pin12: 2.49V   Pin13: 2.49V   Pin14: 2.49V
    Pin15: 2.39V   Pin16: 2.47V   Pin17: 2.48V   Pin18: 0.00V

The voltage drop on Pin 15 may result from the high internal impedance of this signal (OSC2/CLKOUT).

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