Boot Mode Control; Microprocessor 7.3975 Mhz Clock; Battery Gauge; Audio Circuitry - Motorola EP450 Service Manual

3-4
The microprocessor reset line (pin 94) can be controlled directly by the digital 3.3 V regulator (U320
pin 7), the microphone jack (part of accessory connector J471) via Q472 and Q471, and the
microprocessor itself. U320 pulls the reset line low if the digital 3.3 V source loses regulation. This
prevents possible MOS latch-up or overwriting of registers in the microprocessor because the reset
line is higher in voltage than the microprocessor VDD ports (U401 pins 12, 39, 59, 88). The
microprocessor can drive the reset line low if it detects a fault condition such as an expired watchdog
timer, software attempting to execute an infinite loop, unplanned hardware inputs, static discharge,
etc. Finally, the Q471 can pull the reset line low during use of the programming cable and CPS by
the application of a sufficiently negative voltage to the microphone connector tip contact (J471 pin 4),
however this reset method is not utilized.

3.1.1.8 Boot Mode Control

When power-up reset occurs, the microprocessor will boot into either normal or flash mode
depending on the logic level of ports MODA (U401 pin 58) and MODB (pin 57). The Flash Adapter is
a programming accessory which provides negative 9 volts dc via a 1K resistor to microphone
connector J471 pin 4. This turns on Q471 and Q472 via D471 and VR472, pulling MODA and MODB
low and allowing booting in the flash mode by cycling power to reset the radio. Software upgrades
can then performed by loading the new software code into Flash ROM U404.

3.1.1.9 Microprocessor 7.3975 MHz Clock

The 7.3975 MHz clock signal (uP_CLK) is provided from the ASFIC_CMP (U451 pin 28). Upon
startup the 16.8MHz crystal provides the signal to the ASFIC_CMP, which sends out the uP_CLK at
3.8MHz until a steady-state condition is reached and the clock is increased to 7.3975MHz for the
microprocessor.

3.1.1.10 Battery Gauge

Various battery types are available having different capacities. The different battery types contain
internal resistors connected from the BATT_CHARGE contact to ground (which is routed to the
microprocessor as BATT_DETECT). A voltage divider is formed with R255 producing a different DC
voltage for each battery type, which is read by microprocessor port PE2 (pin 65). This allows the
software to recognize the battery chemistry being used and adjust the battery gauge for best
accuracy.
3.1.2

Audio Circuitry

3.1.2.1 Transmit and Receive Low-Level Audio Circuitry

The majority of RX and TX audio processing is performed by U451, the Audio Filter IC
(ASFIC_CMP), which provides the following functions:
• Tone PL/Digital PL encode and decode filtering
• Tone PL/Digital PL rejection filter in RX audio path
• TX pre-emphasis amplifier
• TX audio modulation limiter
• Post-limiter (splatter) filter
• TX deviation adjust (digitally-controlled attenuators)
• Programmable microphone gain attenuator
• RX audio volume control (digitally controlled attenuator)
• Carrier squelch adjustment (digitally controlled attenuator)
• Microprocessor output port expansion
November 24, 2003
Controller Theory of Operation: Controller
HKLN4216A