Elecraft KDSP2 Manual - Manual (Page 43)

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Auxbus Interface
The K2 auxbus operates at a nominal 5V, but may be as high as 6V with some accessories. This is
incompatible with the 5V microcontroller used in the KDSP2. Level translation is accomplished by Q1-Q3
and associated circuitry.
Incoming auxbus data is filtered by C12 and applied via R8 and D2 to an input pin on the microcontroller,
U1. R9 serves as a weak pull-up for this input. D2 is a Shottky diode, having very low voltage drop. It is
used to couple the low-level (logic “0”) signals into U1. D2 and R9 combine to limit the input high levels
(logic “1”) to +5V. This protects U1 from the potential +6V levels on the auxbus.
Q1 and Q3 are normally biased off by a LOW output from U1 to their respective bases through R3 and R4,
respectively. Q1 being off allows R2 to bias Q2 off. As a result, the auxbus drivers from the KDSP2 are in
the OFF, or high-impedance, state.
If the KDSP2 needs to assert an auxbus level of HIGH, it turns on Q1, which turns on Q2 and pulls R7 to
+5 VDC. This high level is passed through R6 and R7 to the auxbus.
If the KDSP2 needs to assert an auxbus level of LOW, it turns on Q3, whose collector is tied to the auxbus
through R6 and R8.
The microcontroller always turns OFF both Q1 and Q3 before turning either ON, to prevent contention.
R6, R7 and R8 serve as current limiters in case of an auxbus fault.
Microcontroller
U1 includes a CPU, Flash program memory, EEPROM and RAM, along with I/O ports and timers. Flash
stores the program, EEPROM the constants and filter settings.
Z1 controls the clock of U1, setting it to a nominal 4 MHz, the same as most other microcontrollers in the
K2.
U1 monitors auxbus activity, interprets button presses, manages the menus and displays, and controls the
DSPx Module.
Because the DSPx module operates internally at 3.3V (see below), it must be protected from the 5V output
of U1. Shottky diode D3 provides this isolation, allowing U1 to pull J1 pins 11 and 12 to near ground, but
preventing them from being driven to 5V. A pull-up resistor to +3.3V is included in the DSPx module to
limit the applied level when U1 pin 3 goes high.
The LED is operated directly by the DSPx Module via Q4, and is used for status information.