Elecraft K2 Owner's Manual - Manual (Page 114)

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LECRAFT ® 113
Potentiometers R1, R2 and R5 (Keyer Speed, Power Out, and RIT/XIT
Offset) are multiplexed onto a single A-to-D input of the MCU, the "VPOTS"
line, so their position can be read. Firmware hysteresis is used for these
controls to prevent noise from interfering with the readings, with more
hysteresis on transmit. The AF GAIN control is not read by the MCU; its
leads go directly to the input of the AF amp on the Control Board. (The entire
path from product detector to AF amp is balanced to prevent common-mode
noise pickup—see Control Board for details.) As is true of most modern
transceivers, the RF GAIN control actually controls the receiver’s IF gain; it
varies the DC control voltage on pin 5 of U12 (RF Board).
The circuitry associated with J2, the mic jack, is only present if the SSB
option is installed. P1 is a configuration header that the user can wire as
needed to support any of several industry-standard microphones with an 8-pin
circular connector. Q3 and its associated resistors are used to multiplex the
UP, DOWN, and FUNCTION lines from P1 onto the VPOTS line to allow
the mic to send commands to the MCU. The PTT line from the MIC activates
the DOT-PTT line to initiate transmit. The MICAF line, mic audio output, is
amplified and processed by circuitry on the SSB adapter (see Option
Modules).
Control Board
The Control board plugs into the RF board via connectors P1, P2, and P3
(along the bottom edge of the schematic). P1 handles the AGC signals while
P2 provide miscellaneous I/O. Redundant connections are provided for
ground, supply voltages, low-impedance signals (such as audio output) and a
few other critical signals.
U6 is a PIC18C452 microcontroller (MCU), with 8 k of EPROM, 300+ bytes
RAM, serial I/O, parallel I/O, and A-to-D inputs. It is self-contained with the
exception of its 4 MHz crystal oscillator, X2. Even when running at 4 MHz,
the PIC processor is very efficient: it only draws a few milliamps at 5 V.
Also, since the program and data memories are located on-chip, there is very
little noise radiation from the MCU.
To get the most out of the available I/O on the MCU, much of the
communication from MCU to the rest of the K2 is done via serial interfaces:
RS232: Used for communicating with a computer via P4
(Aux I/O)
I2C: Display driver data
SPI: The serial peripheral interface is used to access various
peripherals, including the PLL and DACs.
AuxBus: 1-wire data network for co-processor control
Shift registers: serial-to-parallel shift registers are used to access
MOSFET LED drivers on the front panel; a parallel-to-
serial shift register on the front panel is used for reading
pushbuttons.
In addition to the microcontroller the Control board provides a number of
specialized hardware interfaces. Circuitry is described moving from left to
right, top to bottom on the schematic.
U10A and associated circuitry are used to accurately control power output as
well as provide CW waveform shaping. The keying waveform is sigmoidal
(S-shaped) on both rising and falling edges in order to provide totally click-
free keying.
Q9 and Q10 form a two-stage amplifier, supplying a square wave signal to
the MCU when the frequency counter is enabled and a probe is connected to
P6. The counter amp is turned off at all times except when one of the
calibration routines is being used.
The four outputs from the quad DAC (U8) provide: audio tones (via U10B),
BFO frequency control (U10D), and crystal filter bandwidth control (U10C).
Audio tone pitch, amplitude, and wave shape are controlled in firmware to
yield clean sidetone from 400-800 Hz, as well as general-purpose tones. The
bandwidth control line doubles as the transmitter driver bias control on
transmit.
Note: The sidetone signal is actually generated at pin 4 of U8, which is a
logic output, while sidetone volume is set by a D-to-A output of the DAC
using Q5 as a variable-drain-voltage saturated switch. The DAC cannot be
used to generate sidetone directly because the 60 dB channel-to-channel
isolation is not adequate to prevent slight modulation of the VBFO and
BVIAS lines on transmit.