Motorola MPC821FADS User Manual page 9

Installation
2.1.2 Selecting the Power-On Reset Source
The functionality of the power-on reset logic changes with each revision of the MPC821. For
your purposes, this means that you may need to select a different source to generate a
power-on reset. To select your power-on reset source, you need to set J1 on the
MPC821FADS daughterboard. When the J1 jumper is between positions 1 and 2, a
power-on reset is generated by the keep-alive power rail (KAPWR). For example, when
KAPWR goes below 2.005V, a power-on reset is generated. When the J1 jumper is between
positions 2 and 3, a power-on reset is generated from the main 3.3V power rail. In other
words, when the 3.3V power rail gets below 2.805V, a power-on reset is generated.
2.1.3 Selecting the Keep-Alive Power Source
The J2 jumper is used to select the keep-alive power source. When the J2 jumper is
between positions 1 and 2, the keep-alive power is fed from the main 3.3V bus. When you
need to connect an external power source (such as a battery) to the keep-alive power rail,
it should be connected between positions 2 (the positive pole) and 3 (GND).
3.3V
KAPWR
GND
2-4
J1
1
KEEP-ALIVE POWER RAIL
Figure 2-3. Power-On Reset Source
J2
1 3.3V
KAPWR
GND
KAPWR FROM 3.3V KAPWR FROM EXTERNAL
Figure 2-4. Keep-Alive Power Source
MPC821FADS-DB USER'S MANUAL
J1
1
MAIN POWER RAIL
J2
1
POWER SUPPLY
+
–
EXTERNAL
POWER SUPPLY
MOTOROLA