AN2201 Freescale Semiconductor / Motorola, AN2201 Datasheet - Page 4

• Manufacturer Part Number: AN2201
• Description: Low Battery Cranking Pulse in Automotive Applications
• Manufacturer: Freescale Semiconductor / Motorola

AN2201/D
4
NOTE:
Provided an appropriate external reset signal is applied to the MCU, preventing
the CPU from executing code when VDD5 is out of specification limits, the
SRAM content’s integrity is guaranteed, if after a reset has occurred, the PORF
bit in the CRG flags register has not been set (taken from HCS12
Specification).
Figure 4
advantages of the Motorola system basis chip (SBC) MC33389 in combination
with a member of the HCS12 family, the latest Motorola 16-Bit Microcontrollers
(for example, the MC9S12DP256).
Low Battery Cranking Pulse in Automotive Applications
1. VBat falling
2. Early warning
3. MCU sees RESET
4. MCU resumes normal operation, PORF check
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Note that there is a difference between the VBat (actual Battery Voltage)
and VBat1 (reduced by a Diode and buffered by a Capacitor)
MCU realizes that the Module supply is breaking down --> MCU saves
data and enters power saving mode
(Example: Pseudo Stop + cyclic wake up, thus the MCU can monitor the
voltage and can resume work when the voltage is rising again without
going through reset)
As the voltage is dropping further down, the MCU is being put in reset
state
Note that this also puts all I/Os and registers in predefined states.
The MCU voltage is high enough to allow the MCU to operate properly,
so reset is being removed. The MCU software may check the power on
flag (PORF) which indicates whether the RAM data integrity is good or
a complete initialization sequence is needed.
shows a possible power supply for the module, utilizing the
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