QRP Labs QCX 5W CW, Руководство по сборке, Страница: 119 / 138

QCX assembly Rev 1.08
119
In all three cases, the voltage when a button is pressed, is higher than the 3V logic “1” threshold of
the microcontroller. I am able to use a pin-change interrupt to detect that a button has been
pressed, then read the ADC channel 3 and compare the measured voltage to the table, to
determine which button was pressed.
It’s a nice trick for reading multiple buttons on a single ADC input, and it would work also for a
larger number of buttons! Each one just needs a different resistor value and eventually, some care
would need to be taken to ensure that the voltage differences could be read reliably, not masked
by ADC noise or component inaccuracies. For three buttons, none of these problems arise.
TX/RX Switch control, and Key Out signals
As already discussed, the Key Out signal either reflects directly the state of a straight key, in
Straight keyer mode, or it is a processed keyer signal when using the Iambic keyer modes or
Beacon function of the transceiver. The Key Out signal is generated by the microcontroller with
precise timing in the Beacon and Iambic keyer modes. It is routed to the RF envelope key-shaping
circuit discussed previously.
The Transmit/Receive switch must remain in “transmit”
state for a short time after key-up, in order for the RF
envelope of the key-shaping circuit to drop gently
down to zero, which takes approximately 5ms. Due to
this, the microcontroller implements a 10ms delay
between key-up and switching the radio back to
receive mode.
The diagram (right) illustrates the relationship between
these signals, for an example 24 words per minute
“dit”. The Key out signal has a “high” period of 50ms
exactly. The ramp-up and ramp-down of the RF
envelope has 5 millisecond (approximately) rise and
fall times. The “RX” control line generated by the
microcontroller switches on the receiver (allows RF
through to the receive mixer) when it is “high”. It is
held in the “low” (transmit) state for 10ms after key up
completes.
The “TX” signal is the inverse of the “RX” signal, and it switches the gate of the audio mute switch
Q7 during transmit.
One of the NAND gates IC3D is used as the inverter to create the “TX” signal
from the microcontroller’s “RX” output.
5.14 Optional GPS interface
The GPS was a late addition to the design. It is added to facilitate
several useful features:
•
Calibration of the 27MHz reference oscillator
•
Calibration of the 20MHz system clock
•
For WSPR, discipline of the frequency (drift correction)
•
For WSPR, setting and discipline of the real time clock
•
For WSPR, setting the Maidenhead Locator square