Vox Operation; Programming And Flashing Through Microphone Jack - Motorola EP450 Service Manual

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3.1.2.5 VOX Operation

VOX audio accessories do not have a PTT switch. Instead, the mic cartridge is wired directly from
J471-4 to ground. If the radio has been programmed for VOX operation and the VOX accessory is
plugged in prior to turning the radio on, the current drawn by the cartridge will turn on Q470 (pins 3-
4-5) and a logic high will be seen at port PJ1 at turn-on. The microprocessor then assumes VOX
operation, with PTT controlled by the presence of audio at the mic cartridge. A dc voltage
proportional to the audio level at the input of the ASFIC_CMP (U451 pin 46) is fed to an A/D input of
microprocessor U401 (pin 62). During VOX operation, PTT is activated when the dc level exceeds a
preset threshold.

3.1.2.6 Programming and Flashing Through Microphone Jack

The ring contact on the 2.5 mm microphone jack is used for reading, programming or re-flashing the
radio using CPS. This contact (J471 pin 6) is routed to ports PD0_RXD (data into uP, pin 97) and
PD1_TXD (data out of uP, pin 98). Transistor Q410 isolates the input and output functions by
allowing PD1 to pull the line low, but does not affect incoming data from being read by port PD0.
To re-flash the radio (overwrite the software in the Flash ROM with new software), the radio must
power up in the boot mode. This is accomplished by using a flash adapter accessory, which provides
SCI communication with the programming ring contact (J471 pin 6) and also allows a negative
voltage (negative 9 volts dc via a 1K resistor) to be applied to the tip contact (J471 pin 4). This
voltage is sufficient to turn on the base-emitter junction (pins 1 and 2) of Q472 via L471, D471,
VR472 and R471. Pin 6 of Q472 goes high, turning on Q471 (pins 3 and 4) and pulling the
BOOT_ENA line (ports MODA and MODB of the microprocessor) low. Cycling power generates a
reset which causes the radio to boot in the flash mode.
November 24, 2003
Controller Theory of Operation: Controller
HKLN4216A