Chapter 3.5 900 MHz Radio Power
3.5.1 Introduction
This chapter of the manual provides a detailed circuit description of the power distribution for an
ASTRO XTS 2500/XTS 2500
I digital portable radio. This manual is to be used in conjunction with the
ASTRO XTS 2500/XTS 2500
I Digital Portable Radios Basic Service Manual (Motorola part number
6816984H01), which uses the pass/fail service approach to radio problems.
3.5.2 Radio Power Up or Down
Radio power up begins when the radio ON/OFF/Volume control knob is turned to the on position,
placing 7.5 volts on the MECH_SW line. This signal activates the pass element (Q700, Q701 and
Q703), enabling SW_B+ and RFSW_B+.
When the MAKO IC (U701) detects a low to high voltage transition on the MECH_SW pin, it grounds
the FET_ENX pin (or VPP_EN) driving the gate of an external P-MOSFET (Q700 and Q703) low.
This turns ON the transistors Q700 and Q703 and applies SW_B+ to MAKO IC and VoCon and
RF_SWB+ to the RF section of the board. After application of the power to SW_B+, the regulators
VSW1, VSW2, V1.875, V1.55, V2.9 and VCC5 are sequenced on. After the regulators are stable, the
RESETX pin is asserted low for an additional 16ms and then de-asserted allowing the dual-core
processor to start.
The radio power down sequence begins by opening the radio ON/OFF switch, causing the
MECH_SW pin to transition high. This causes the MAKO IC to start an internal 125ms watchdog
timer and drive the INT_X pin (or MAKO_INT) interrupt output pin low. The dual-core processor then
determines (through communication on the SPI bus) that it must initiate the power-down sequence.
At this point, the watchdog timer can be kept alive by the dual-core processor (via the SPI bus)
allowing for software cleanup. Upon expiration of the watchdog timer, the MAKO IC asserts the
RESETX pin and turns off all DC regulators including SW_B+ and RF_SWB+ (Q700 and Q703 turn
off when the MAKO IC drives the FET_ENX pin high).
3.5.3 General
In the XTS 2500/XTS 2500I radio, power (B+) is distributed to one board that contains both the
transceiver and controller sections. There is an additional backup battery (Motorola part number
6003710K08) mounted directly on the board; see the basic service manual for details.
Power for the radio is provided through a battery supplying a nominal 7.5Vdc directly to the
transceiver. The battery is available in the following forms:
• NTN9815_ Nickel-Cadmium, Hi Capacity
• NTN9816_ Nickel-Cadmium, Hi Capacity, FM Approved
• NNTN9857_ Jedray NiMH Impres Standard Battery
• NNTN9858_ Jedray NiMH Impres FM Battery
B+ from the battery is electrically switched to most of the radio, rather than routed through the ON/
OFF/Volume control knob, S501. The electrical switching of B+ supports a “keep-alive” mode. Under
software control, even when the ON/OFF/Volume control knob has been turned to the “off” position,
power remains on until the MCU completes its power-down, at which time the radio is physically
powered-down.
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