MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor
23-3
23.4
Functional Description
The VRC portion of the module contains a voltage regulator controller, and the POR portion contains
circuits to monitor the voltage levels of the 1.5 V and VDDSYN supplies as well as circuits to monitor the
supply that powers the RESET pad. The PORs indicate whether each monitored supply is above a specified
voltage threshold. These PORs are used to ensure that the device is correctly powered up during a
power-on reset. The MPC5553/MPC5554 resets the device if any of the supplies are below the specified
minimum.
23.4.1
Voltage Regulator Controller
The VRC circuit provides a control current that can be used with an external NPN transistor and an
external resistor to provide the 1.5 V V
DD
supply. The control current is output on the V
RCCTL
pin. The
voltage regulator controller begins to turn the pass transistor on slowly while the 3.3 V POR still is
asserted. The pass transistor will be completely turned on when the 3.3 V POR negates.
NOTE
The voltage regulator controller will keep the 1.5V supply in regulation as
long as V
RC33
is in regulation. If more protection is desired, the customer
may also supply an external 1.5 V low voltage reset circuit.
If the on chip voltage regulator controller is not used, an external 1.5 V
power supply must be used. To avoid a power sequencing requirement when
an external power supply is used, external 3.3 V must power V
RC33
while
the V
RCCTL
pad is unconnected. In this case the internal 1.5 V POR will
remain enabled. If the V
RC33
is not powered, the device is subject to power
sequencing requirements for the 1.5 V and 3.3 V or RESET power supplies
(See
Section 23.5.3, “Power Sequencing
”). This is necessary to ensure that
the 1.5V power supply is high enough for internal logic to operate properly
during power-up.
23.4.2
POR Circuits
The individual POR circuits will negate whenever the supply they are monitoring is below a specified
threshold. The entire device will be in power-on reset if any of these supplies are below the values
specified in the
MPC5553/MPC5554 Microcontroller Data Sheet
.
Power-on reset will assert as soon as possible after the voltage level of the POR power supplies begins to
rise. Each POR will negate
before
its power supply rises into its specified range. Each POR will assert
after
its power supply drops below its specified range. Power-on reset will remain asserted until all of the POR
power supplies have dropped below the minimum POR threshold. The behavior for each POR during
power sequencing is shown in
Before the 3.3 V POR circuit asserts when ramping up or after it negates when ramping down, the device
can exit POR but still be in system reset. In this case, MDO[0] will be driving high. Also in this case,
though, no clocks will be toggling. If the 3.3 V POR circuit is asserted, the device will behave as if in POR
even if the 1.5 V and RESET power POR circuits have not yet asserted when ramping up or have negated
when ramping down.
Summary of Contents for MPC5553
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