ScanTool STN11xx - Manual (Page 5)

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1.3.1 UART Rx Pulse Wakeup
STN11xx can be configured to wake up on an
active pulse detected on the UART Rx input. The host
can generate the pulse by holding the Rx line in a logic
low state, transmitting an RS232 “break” signal, or
sending a character on UART whose bit pattern
produces a pulse of the required duration.
The wakeup pulse has minimum and maximum
timing requirements, which are set using the
STSLUWP command, and are accurate to within
approximately 5 μs. By default, the minimum wakeup
pulse width
is set to 0, which translates to an absolute
minimum pulse width requirement of 20 ns. It can be
increased to improve noise rejection; however,
increasing the minimum pulse width will limit the
maximum baud rate that the host must use to transmit
the wake-up character. Due to the implementation
limitations, setting the minimum wakeup pulse width to
any value below 15 μs will cause it to be rounded down
to 0 (20 ns).
The purpose of the maximum wakeup pulse
width requirement is to avoid unintentional wakeups.
Some PC hosts (especially ones using the RS232
connection) cause the UART Rx line to go low or
generate a slow (200 ms or longer) pulse as the host is
shutting down or entering standby. The default setting
is 30 ms, which allows the device to wake up on a
character transmitted over UART at baud rates as low
as 300
baud. To disable the maximum pulse
requirement and have STN11xx wake up on the high to
low UART Rx transition (instead of a pulse), set the
maximum pulse timing setting to 0.
1.3.2 External SLEEP Input Wakeup
STN11xx can be configured to wake up when it
senses logic high on the external SLEEP control input.
The STXWT commands sets the minimum time the
SLEEP input must remain high in order to bring the
device out of the sleep state. The setting of 0 will result
in a minimum time requirement of 15 μs.
Note: STN1110 and microOBD 200 (STN1120)
allow the polarity of the external SLEEP input to be
inverted, via the STSLXP command.
Section 1.2.3 lists possible applications for the
external SLEEP input.
1.3.3 Voltage Level Wakeup
The STN11xx can also wake up based on the
voltage on the ANALOG_IN input. This trigger can be
configured in a variety of ways. The settings specify
voltage threshold, whether the trigger is active above
or below the threshold setting, and the minimum
amount of time the voltage must stay below or above
the threshold for the device to wake up.
The voltage level wakeup trigger can be used to
wake up the device when the engine starts up and the
alternator causes the system voltage to increase. The
default settings should work for most vehicles with
lead-acid batteries.
Use the STSLCS command to verify that the trigger
was properly configured. An exclamation point (‘!’) in
front of the voltage setting means that the trigger
setting is invalid, and the trigger will never activate.
Refer to section 1.4 “Voltage Trigger Considerations”
for more information.
Note: In order for the voltage level wakeup trigger
to operate properly on STN1110 IC or microOBD 200
module (STN1120), the voltage measurement must be
calibrated using the ATCV or STVCAL commands. The
calibration is not necessary for the OBDLink scan
tools, since it is done at the factory.
1.3.4 Voltage Change Wakeup
The STN11xx can be configured to wake up when
the difference between two consecutive voltage
samples taken at the ANALOG_IN input exceeds a
predefined threshold. The settings specify polarity of
the change (rising, falling, or either), the change in
volts or ADC steps, and the time between the samples.
The voltage level wakeup trigger can be used to
wake up the device when the starter motor is cranking
the engine (battery voltage dips) or when the engine
starts up (voltage rises due to alternator running). This
wakeup trigger can be more reliable than the voltage
level wakeup trigger, since it does not rely on a specific
voltage level which can vary between vehicles, but
instead detects voltage change, which happens every
time engine starts no matter what the battery level or
the alternator voltage is.
Use the STSLCS command to verify that the trigger
was properly configured. An exclamation point (‘!’) in
front of the voltage setting means that the trigger
setting is invalid, and the trigger will never activate.
Refer to section 1.4 “Voltage Trigger Considerations”
for more information.
Note: If a non-default voltage scaling is used for the
STN1110 IC or the microOBD 200 module (STN1120),
the voltage measurement must be calibrated using the
ATCV or STVCAL commands, for the voltage change
wakeup trigger to operate properly.
1.4 Voltage Trigger Considerations
Analog voltage that STN11xx “sees” on the
ANALOG_IN pin is represented internally by a 12-bit
integer. The conversion is done by an internal Analog
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