Technical Note
Sleep Mode for Use with Honeywell Digital Pressure Sensors:
ASDX Series, APB Series, and Trustability
®
HSC, SSC Series
Sensing and Control
1.0 PURPOSE
The purpose of this technical note is to provide the information
necessary for using Sleep Mode in the ASDX Series, ABP Series
and TruStability™ HSC Series and SSC Series Digital Output
Pressure Sensor products.
2.0 OVERVIEW
Sleep Mode is a special factory set mode of the pressure sensor
that allows the sensor to power down between necessary
pressure readings. By implementing this mode, current
consumption during the non-measurement time of the sensor
can be reduced from 2.5 mA typ. to 0.5 μA typ.
This is ideal for applications where current consumption is more
important than speed, such as battery-powered applications.
The sensor only “wakes up” when sent a special command over
the I
2
C or SPI digital communication buses, depending on the
type of digital output sensor.
3.0 DESCRIPTION (see Figure 1)
In Sleep Mode, following the power up sequence, the
sensor will power down again until the master sends a “Full
Measurement Request” (either I
2
C or SPI) or a “Pressure
Measurement Request” (I
2
C only) command. Specifics on the
Full Measurement Request (FMR) and Pressure Measurement
Request (PMR) commands are given in Section 4.0.
A FMR or PMR wakes the sensor and starts a measurement
cycle.
• FMR: The sensor performs a full measurement and
calculation cycle, including all secondary measurements
(temperature and A/D auto-zero).
• PMR: The sensor measures only the sensor bridge and
performs the correction calculations using previously
measured temperature and auto-zero data. Valid output
values are then written to the digital output register and the
sensor powers down again.
Following a measurement sequence, and before the next
measurement can be performed, the master must send a Read
Data Fetch (RDF) command, which will fetch the data as 2, 3
or 4 bytes (see Section 4.0), without waking the sensor. When
an RDF is performed, the data packet returned will be the last
measurement made with the status bits set to “valid”. After
the RDF is completed, the status bits will be set to “stale”. The
next FMR or PMR will wake the sensor again and start a new
measurement cycle.
Figure 1. System Timing
Power Down
Measure
Bridge
Measure
Auto-zero
Measure
Temp.
Calcula-
tions
Measure
Temp.
FMR
Full
Measurement
Request
RDF
Read
Data
Fetch
RDF
Read
Data
Fetch
Sensor core activity
Digital interface activity
Approx. 1.26 ms
Read here returns
invalid data
Read here returns
valid data
StartupPOR
FMR wakes
sensor
FMR wakes
sensor
Power Down
Power Down
Power Down
Measure
Bridge
Calcula-
tions
Measure
Temp.
RDF
Read
Data
Fetch
RDF
Read
Data
Fetch
Sensor core activity
Digital interface activity
Approx. 460 us
Read here returns
invalid data
Read here returns
valid data
StartupPOR
PMR wakes
sensor
PMR wakes
sensor
Power Down
Power Down
Full
Measurement
Request
Pressure
Measurement
Request
FMR
Full
Measurement
Request
PMR
Pressure
Measurement
Request
PMR
Pressure
Measurement
Request
2 sensing.honeywell.com
Technical Note
Sleep Mode for Use with Honeywell Digital Pressure Sensors:
ASDX Series, APB Series, and Trustability
®
HSC, SSC Series
If an RDF is sent while the measurement cycle is still in progress,
then the status bits of the packet will read as “stale”. The sensor
should be polled at a frequency slower than 20% more than the
Sleep Mode response shown in Table 1.
Table 1. Sleep Mode Response Times
1
Measurement Request Response Time
Full Measurement Request (FMR) 1.5 ms
Pressure Measurement Request (PMR) 0.5 ms
Note 1. Times are typical. For worst case, multiply by 1.15 (15%
frequency variation).
In I
2
C mode only, the INT pin of the sensor will assume an
interrupt function. This allows the user to monitor the interrupt
pin instead of continually polling the output looking for valid
data. The interrupt will indicate that the measurement and
calculations are complete, and that new valid data is ready to be
read on the I
2
C interface. (This function is only available for DIP
and SMT versions.)
4.0 SLEEP MODE COMMANDS
4.1. I
2
C Measurement Request Commands
4.1.1. Full Measurement Request (FMR)
The command structure shown in Figure 2 is used to wake the
sensor from sleep mode and perform a full measurement cycle,
including secondary measurements.
Figure 2. Command Structure for I
2
C FMR
The command begins with a Start bit, followed by the seven
bit address, followed by a “1”, indicating a Read. The master
then waits for the sensor to generate an Acknowledge, and then
immediately generates a Stop condition.
4.1.2. Pressure Measurement Request (PMR) Command
The command structure shown in Figure 3 is used to wake the
sensor from sleep mode and to perform a partial measurement
cycle without secondary measurements. This measurement is
faster than the FMR but potentially less accurate because it uses
previously stored values for temperature and A/D auto-zero.
Figure 3. Command Structure for I
2
C PMR
The command begins with a Start bit, followed by the seven
bit address, followed by a “0”, indicating a Write. The master
then waits for the sensor to generate an Acknowledge, and then
immediately generates a Stop condition.
4.2. SPI Full Measurement Request (FMR) Command
Because the SPI protocol is only half duplex, there are no Write
conditions for the bus. Because of this, only the FMR command
is available in SPI mode. Also, because of the read-only nature
of SPI for Honeywell pressure sensors, the command is
executed in a different manner.
The SPI FMR only requires that the SS line be dropped low for
a minimum of 8 μs, then raised high again. The rise of SS will
trigger the sensor to power up and perform the measurements
(see Figure 4).
Figure 4. SPI FMR Command
A4A5 A3 A2 A1 A0 0A6
WriteSlave Address [6:0]
Start
ACK
STOP
Bits generated by master
Bits generated by slave (sensor)
SS
8 μs
A4A5 A3 A2 A1 A0 1A6
ReadSlave Address [6:0]
Start
ACK
STOP
Bits generated by master
Bits generated by slave (sensor)