Reset Circuits; Figure 3-46. Power-On Reset Circuit - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

Theory of Operation: Controller Section
3.8.4

Reset Circuits

The reset circuits consist of the power-on reset (POR*) circuit
and SB9600 bus reset circuit. These circuits allow the microprocessor to recover from an unstable
condition, such as removing battery A+ from the radio while it is on, battery voltage too low, and
miscommunication to remote devices on the SB9600 bus, as well as generally monitoring the power
on/off condition.
SW_B+
Soft_Turn_Off
Emergency
SW_A+
SW_B+
5V
4.2V
Detector
U05
The SW_B+ and A+ voltage levels are sensed by the comparator circuit consisting of U0604, Q0505,
and R0505, R0506, R0508, and R0509. When SW_B+ goes below 8.5 V or SW_A+ goes below
10 V, U0604-7, SW B+ Sense, goes low. When this occurs, the radio completes its soft power-down
and eventually drives the Soft_Turn_Off line low, which turns off Q0502 and FET Q0503. This turns
off SW_A+ and eventually turns off the 9-V, 5-V, 3-V, 2.85-V, 1.85-V, and 1.55-V regulators.
The POR* circuit consists of a wired-OR circuit of the error output lines from the 2.85-V, 1.85-V, and
1.55-V regulators, which indicates a failure of either of these regulators; and a reset output from a
4.2-V detector IC U0504. When either of the regulators fails or the 5-V supply begins to drop below
4.2 V, POR* is asserted low, resetting the microprocessor U100 at U001, LV_DETECT.
When the radio is operating correctly (A+ > 10 V and SW_B+ > 8.5 V), SW_A+ and VOCON
regulators U0501, U0502, and U05007 are at normal voltage. The POR* line and SW_B+_Sense are
high (2.85 V).
6881096C74-B
Q0505
U0604
+
SWB+ _Sense
-
2.85V 1.85V
Reg. Out Reg. Out
U0501 U0507

Figure 3-46. Power-On Reset Circuit

(Figure 3-46), SW_B+ sense circuit,
A+
FET
B+_CNTL
To Microprocessor
2.85V
(LV_DETECT)
1.55V
Reg. Out
U0502
SW_A+
To 9.3V Regulators
(U0500......)
POR
To Microprocessor
MAEPF-27826-O
May 25, 2005
3-67