THEORY OF OPERATION
SST-442
20
MICROCONTROLLER
The SST-442 handheld transceiver is electronically
controlled by IC301, an 8-bit microcontroller. IC301
has four 8-bit A/D inputs for processing analog
signals.
PIN DESCRIPTION
1 RESET is connected to the SST-442 +5V to start
the radio in a known state on power up.
2 TX KEY input is pulled LOW when either the
internal or external PTT switch is pressed to
initiate transmitter operation.
3 AUDIO GATE ENABLE controls analog switches
IC304A and B to configure the shared audio
processing circuitry for receive operation when
LOW, and for transmitter operation when HIGH.
4
SERIAL DATA INPUT links the microcontroller to
communications from an external data terminal
via J303 RING connection. This allows
programming of the SST-442 EEPROM memory
used to store channel frequency and
configuration information.
5
DATA output sends serial data to frequency
synthesizer IC401 to program frequency
information, IC302 EEPROM memory, and to
IC306 to set the digital potentiometers.
6
EECLOCK output sends serial data clock pulses
to EEPROM IC302 when reading or writing.
7
LOW-PASS CUT-OFF output goes LOW to lower
the cut-off frequency of IC308 when the radio is
on a channel programmed for a CTCSS tone
below 150 Hz. This output is in a tri-state OPEN
condition for all other tones.
8
DIGITAL POT LATCH sends a single positive
pulse after data has been sent to IC306 to latch
the new data and change the digital
potentiometers to the new programmed settings.
9
LED output goes LOW when the radio is on
channel 1 and is HIGH for all other channels.
10 +5V SW output goes HIGH at a programmable
periodic rate to remove the +5VSW supply from
various circuits in the radio, thus conserving
battery life. In normal transmit or receive mode
this output is LOW.
11 T/R SWITCH output is connected to the
Synthesizer circuitry to shift the frequency of the
VCO oscillator used in both transmit and receive.
The output is HIGH in transmit and LOW in
receive. T/R output is also used to switch supply
voltage to the transmit and receive circuits.
12 CLOCK output sends serial data clock pulses to
frequency synthesizer IC401 and digital
potentiometers IC306 when programming these
devices.
13 +V SW output is HIGH when the radio is turned
on, keeping pass transistor Q304 turned on via
Q303 to supply power to the radio.
14 GROUND
15 V
REFH
sets the upper reference level for the A/D
and is connected to the reg5 VDC.
16 A/D input BATTERY is used to measure the
battery voltage for low and dead battery
detection. This input also serves as the ON/VOL
UP input, and is pulled LOW when SW301 is
depressed to turn on the radio and raise the
receiver speaker audio level.
17 A/D input VOL DN/OFF is pulled LOW when
SW302 is depressed to lower the receiver
speaker audio level and turn off the radio. This
input also serves as CHANNEL input, and goes
to 2.5 VDC when channel selector switch SW303
is pressed to change the radio to the next
programmed channel.
18 A/D input TONE DECODE accepts the received
QC (CTCSS) and DQC (DCS) waveforms after
signal processing for decode analysis.
19 A/D input RSSI is used to measure the output
voltage of the noise filter for squelch detection.
20 LATCH output goes LOW to allow serial data to
frequency synthesizer IC401 and goes HIGH to
latch the data, allowing the synthesizer to
execute the new frequency information.
21 TONE ENCODE outputs generate the QC
(CTCSS) and DQC (DCS) encode waveforms for
signal processing prior to modulating the VCO in
transmit mode.
22 AUDIO AMP ENABLE output goes HIGH to apply
power to audio amp IC307 for receiver speaker
audio or radio alert tones.
23 RX AUDIO GATE ENABLE switches IC304C for
receive operation when LOW, and for transmitter
operation when HIGH.
24 BEEP output generate the radio alert tones
heard in the speaker.
25 See Pin 4
27 REFERENCE OSCILLATOR INPUT has the 3.6
MHz reference signal from the synthesizer.
28 +5 VDC V
DD
supply voltage.