ScanTool STN11xx - Manual (Page 6)

STN11xx PowerSave
to Digital Converter (ADC for short). The voltage
represented by a single bit is called an “ADC step”, and
is measured in volts per bit (V/bit).
Since the maximum voltage that can be directly
measured by the ANALOG_IN pin is very low (about
3V), the pin is normally connected to the voltage
source to be measured via an external voltage divider.
As its name implies, the voltage divider outputs a
voltage that is a fraction of the actual input voltage. For
example, a 1:10 voltage divider would output 1.2V for
an input voltage of 12V.
Parameters to the voltage based triggers can be
specified either in volts, or as raw ADC values.
When a parameter is specified in volts, STN11xx
internally converts it to a corresponding ADC value.
The size of the ADC step depends on the ratio of the
voltage divider. By default, it is calibrated for a voltage
divider with a ratio of 1:7.2. Keeping in mind that the
maximum voltage on the ANALOG_IN pin is
approximately equal to V
DD (typically, 3.3V), the
maximum voltage that can be measured by the ADC
using default calibration is about 24V:
3.3V ÷ 7.2 = 23.76V
To use voltage triggers with a voltage divider that
has a different ratio, and to account for parts
tolerances, the device must be calibrated using the
ATCV or STVCAL commands.
When a parameter is specified as a raw ADC value,
calibration must be done in the host software. To get
the size of the ADC step, divide the actual measured
voltage by the ADC value reported by the STVRX
command. For example, if the actual measured voltage
is 12V, and the STVRX command returns 0x7FF, the
size of the ADC step is:
12V ÷ 0x7FF = 0.00586V/bit
To convert voltage to ADC steps, divide it by the
ADC step size. For example, using the values above,
8V is equal to 0x554 steps:
8V ÷ 0.00586 = 0x554
Using ADC values instead of volts eliminates the
need to convert ASCII to floating point and vice versa,
greatly reducing the load on the host processor.
When setting up the voltage triggers, take special
care to make sure that the parameters fall within a
valid range of values. For example, the maximum
voltage that can be represented by an ADC using
default calibration is about 24V. If a parameter is set to
a higher value – say, 25V – it falls outside the valid
range of values.
A calibration change may put a previously valid
value outside of the valid range. For example, if the
calibration reduces the maximum voltage from 24V to
14V, a trigger set at 15V will no longer be inside the
valid range.
Additional examples of invalid settings are a voltage
level trigger specified as ‘below 0V’, and voltage
change trigger parameter set to a value less than one
ADC step size.
Whenever a parameter value falls outside the valid
range, the STSLCS command will display a ‘!’ in front
of the voltage setting to indicate that it is invalid, and
the trigger will never activate.
1.5 External Power Control Output
The PWR_CTRL output can be used to put
external circuitry into a low power mode. This pin
outputs a logic “high” while the device is awake, and a
“low” when STN11xx enters sleep mode.
STN1110 stand-alone IC allows the polarity of the
PWR_CTRL to be changed via the STSLPCP
command or bit 6 of the 0E programmable parameter
(ELM327 LP mode only). The polarity is fixed for all
other STN11xx ICs.
1.6 Device Specific Details
This section describes device-specific PowerSave
implementation details for the different STN11xx-based
ScanTool.net devices.
1.6.1 OBDLink Hardware Rev 1.x
OBDLink devices with hardware revision 1.x have
the following limitations:
• In sleep mode, current consumption is about
37 mA (54 mA if the USB cable is plugged in
and the virtual COM port is closed).
• External SLEEP control input is not enabled
(ATIGN always returns “ON”, and STSLXS
always returns “WAKE”).
• The “STATUS” LED is not controlled by the
STN1100, and remains on during sleep.
1.6.2 OBDLink Hardware Rev 2.0 – 2.4
On OBDLink devices with hardware revisions 2.0-
2.4, the SLEEP input detects voltage on the USB
connector.
When enabled, the SLEEP input trigger can put the
device to sleep when the chip detects that the host is
no longer present. This can happen when the PC shuts
down or hibernates, or when the user unplugs the USB
cable.
Likewise, the SLEEP input can be configured to
wake up the device when the chip detects an active
host.
6 of 13 www.ScanTool.net STN11XXPSB