FUNCTIONAL DEVICE OPERATIONS
OPERATIONAL MODES
After the reset source is removed, the state machine will
drive the RST output low for at least 1ms (typical value)
before driving it high.
In order to select and activate direct wake-up from Lx
inputs, the Wake-up Control Register (WUCR) must be
configured with appropriate LxWE inputs enabled or
disabled. The wake-up input’s state is read through the
Wake-up Status Register (WUSR).
In the 33912, four main reset sources exist:
5V Regulator Low-voltage-Reset (VRSTTH
)
Lx inputs are also used to perform cyclic-sense wake-up.
Note: Selecting an Lx input in the analog multiplexer
before entering low power mode will disable the wake-up
capability of the Lx input
The 5V regulator output VDD is continuously monitored
against brown outs. If the supply monitor detects that the
voltage at the VDD pin has dropped below the reset threshold
VRSTTH the 33912 will issue a reset. In case of over-
temperature, the voltage regulator will be disabled and the
voltage monitoring will issue a VDDOT Flag independently of
the VDD voltage.
Wake-up from Wake-up inputs (L1-L4) with cyclic sense
timer enabled
The SBCLIN can wake-up at the end of a cyclic sense
period if on one of the four wake-up input lines (L1-L4) a state
change occurs. The HSx switch is activated in Sleep or Stop
Modes from an internal timer. Cyclic sense and force wake-
up are exclusive. If cyclic sense is enabled, the force wake-
up can not be enabled.
Window Watchdog Overflow
If the watchdog counter is not properly serviced while its
window is open, the 33912 will detect an MCU software run-
away and will reset the microcontroller.
In order to select and activate the cyclic sense wake-up
from Lx inputs, before entering in low power modes (Stop or
Sleep Modes), the following SPI set-up has to be performed:
Wake-up From Sleep Mode
During Sleep Mode, the 5V regulator is not active, hence
all wake-up requests from Sleep Mode require a power-up/
reset sequence.
In WUCR: select the Lx input to WU-enable.
In HSCR: enable the desired HSx.
• In TIMCR: select the CS/WD bit and determine the
cyclic sense period with CYSTx bits.
External Reset
The 33912 has a bidirectional reset pin which drives the
device to a safe state (same as Reset Mode) for as long as
this pin is held low. The RST pin must be held low long
enough to pass the internal glitch filter and get recognized by
the internal reset circuit. This functionality is also active in
Stop Mode.
• Perform Goto Sleep/Stop command.
Forced Wake-up
The 33912 can wake-up automatically after a
predetermined time spent in Sleep or Stop Mode. Cyclic
sense and Forced wake-up are exclusive. If Forced wake-up
is enabled, the Cyclic Sense can not be enabled.
After the RST pin is released, there is no extra tRST to be
considered.
To determine the wake-up period, the following SPI set-up
has to be sent before entering in low power modes:
WAKE-UP CAPABILITIES
• In TIMCR: select the CS/WD bit and determine the low
power mode period with CYSTx bits.
• In HSCR: all HSx bits must be disabled.
Once entered into one of the low-power modes (Sleep or
Stop) only wake-up sources can bring the device into Normal
Mode operation.
In Stop Mode, a wake-up is signaled to the MCU as an
interrupt, while in Sleep Mode the wake-up is performed by
activating the 5V regulator and resetting the MCU. In both
cases the MCU can detect the wake-up source by accessing
the SPI registers. There is no specific SPI register bit to signal
a CS wake-up or external reset. If necessary this condition is
detected by excluding all other possible wake-up sources.
CS Wake-up
While in Stop Mode, a rising edge on the CS will cause a
wake-up. The CS wake-up does not generate an interrupt,
and is not reported on SPI.
LIN Wake-up
While in the low-power mode, the 33912 monitors the
activity on the LIN bus. A dominant pulse larger than tPROPWL
followed by a dominant to recessive transition will cause a
LIN wake-up. This behavior protects the system from a short
to ground bus condition.
Wake-up from Wake-up inputs (L1-L4) with cyclic sense
disabled
The wake-up lines are dedicated to sense state changes
of external switches and wake-up the MCU (in Sleep or Stop
Mode).
33912
Analog Integrated Circuit Device Data
Freescale Semiconductor
28
FREESCALE [ Freescale ]
