Power On / Off; Ptt Button; On/Off Hook Control - Motorola MCX600E Service Manual

Theory of Operation
2.3

Power On / Off

The On/Off button when pressed switches the radio's voltage regulators on by pulling ON OFF
CONTROL to high via D0971 and connects the base of Q0971 to FLT A+. This transistor pulls the
line ANALOG 3 to low to inform the µP that the On/Off button is pressed. If the radio is switched off,
the µP will switch it on and vice versa. If the On/Off button is pressed and held while the radio is on,
the software detects a low state on line ANALOG 3 and switches the radio off.
While the radio is switched on, the radio's controller enables the regulated output pin 1 of U0971 by
setting line LED CE to high and in turn pulling input U0971-3 to ground by transistor Q0972. Line
LED CE is also used to write data into the shiftregister U0983. After the serial write process line LED
CE is set to low for a few microseconds to update the output of the shiftregister with the new data.
The low - pass filtering provided by C0973 and R0974 filters out these small low periods and
prevents disabling of the voltage regulator output.
2.4

PTT Button

The PTT function is achieved by pressing button S0901 which places a low on the PTT line to the
radio, causing the radio to transmit.
2.5

ON/OFF Hook Control

When the HHCH is off-hook the monitor input (line HOOK; J0901_7) to the radio is open circuit.
When the HHCH is placed on-hook the hang up clip shorts the TP0901 and TP0902 terminals which
takes the monitor input to ground, indicating to the radio that the microphone is on-hook. Diode
VR0922 and C0922, C0923 provide Electro Static Discharge (ESD) and Radio Frequency
Interference (RFI) protection.
B.1-2 Introduction/Theory of Operation