NXP Semiconductors freescale MPC5668E, User Manual, Page: 15 / 54

MPC5668EVB Users Manual Rev 0.1 May 2009
MPC5668EVBUM/D Page 11 of 29
The RESET switch (RED)
and LVI circuitry is located
to the t left of the MCU in
the area titled “RESET “
3.3 Reset Control (Jumper J75)
The EVB incorporates an LVI (Low Voltage Inhibit) device to provide under-voltage protection for the two main
switching regulators (5v and 3.3v). When either of these regulator voltages drops below a certain threshold
level, the LVI will assert the MCU reset line to prevent incorrect operation of the MCU (or EVB circuitry).
The table below shows the approximate threshold voltages for each regulator
Table 3-7 LVI Monitor Threshold Voltages
Regulator Minimum Voltage Before MCU
reset
5.0V Switcher 1.47V
3.3V Switcher 1.47V
The LVI is powered from the 5.0V switching regulator and monitors the 3.3V regulator using a 2
nd
power fail
monitor circuit. The LVI also provides a de-bounced input for EVB reset switch SW5.
Jumpers are provided to disable either the main LVI reset out (which affects the reset from the 5.0V switching
regulator and from the reset switch) or the power fail out circuit (which only affects the reset from the 3.3V
regulator). If the 5v regulator LVI is disabled, the reset switch will not function.
Table 3-8 LVI Control Jumpers
Jumper Position PCB Legend Description
J75
Posn 1-2
FITTED (D)
5.0V switching regulator is monitored, Reset switch
active
REMOVED 5.0V switching regulator is not monitored, Reset
switch inactive
J75
Posn 3-4
FITTED (D) 3.3V switching regulator is monitored
REMOVED 3.3V switching regulator is not monitored
Notes:
-
If the 5.0V switching regulator is disabled for any reason, the LVI circuit will attempt to assert the MCU
Reset signal. Jumper shunts on jumper J20 position 1-2 and 3-4 must be removed in this situation.
This will also leave the reset switch SW5 inoperative.
-
If the 3.3V regulator is disabled, the shunt on jumper J20 position 3-4 must be removed to prevent the
LVI asserting reset.
3.3.1 Reset LEDs
There are two reset LED’s, DS2 (AMBER) and DS3 (RED), placed adjacent to the EVB RESET switch to
indicate the RESET status of the EVB and MCU.
LED DS3, titled “
RST ”, will illuminate if the MCU itself issues a reset. In this condition, LED DS2 will not
illuminate.
LED DS2, titled “
USR ”, will illuminate when one of the following external hardware devices issues a reset to
the MCU:
-
LVI circuitry (either an under-voltage detection or the reset switch is being pressed)
-
There is a reset being asserted from the user connectors or from the daughter card
-
There is a reset being driven from the Nexus or JTAG debug probe
Note that LED DS3 (MCU Reset) will also illuminate during an external (user) reset!