FXTH870xD
Sensors
Freescale Semiconductor, Inc.
81
10.2
Temperature Measurements
The temperature is measured from a
V
B
sensor built into channel 1 of the ADC10 in the same manner as is done in the
FXTH870xD devices with the resulting transfer equation:
Eqn. 3
10.3
Voltage Measurements
Voltage measurements can be made on the internal bandgap to estimate the supply voltage on V
DD
.
10.3.1
Internal Bandgap
An internal bandgap voltage reference is provided to take measurements of the supply voltage. The resulting transfer equation:
Eqn. 4
10.3.2
External Voltages
Measurements of an external voltage on either the PTA0 or PTA1 pins can be made and referenced to the internal bandgap
voltage. The resulting transfer equation:
Eqn. 5
where x = 0, 1 refers to PTA0 or PTA1.
10.4
Optional Acceleration Measurements
The acceleration measurement consists of an interface to an optional acceleration sensing element. Control bits on the MCU
operate the SMI to power up the g-Cell and capture a voltage which is converted by the ADC10. The data from the ADC10 is
then pre-processed by a dynamic range firmware routine that will return the two values necessary to calculate the acceleration,
A
y
, (y = X-axis or Z-axis, depending on selection) in conjunction with values taken from the table in
.
The first value from the firmware routine is the offset step identifier, STEP, with integer values 0 to 15 (i.e. the 16 offset steps).
The other value is the ADC10 data, A
yCODE
, with integer values 0 to 511. A
yCODE
values 1 through 510 are usable; values 0 and
511 indicate fault conditions. The X-axis acceleration is scaled for ~20g range within each of the 16 offset steps, ~10g per step.
The Z-axis acceleration is scaled for ~80g range within each of the 16 offset steps, ~80g or ~60g. The steps are at ~40g or ~30g
increments, allowing for adequate overlaps.
provides a table of acceleration values resulting from
characterizations.
Acceleration sensitivity,
A
y-STEP
, varies between each offset step, and should be calculated by dividing the range of g’s for each
offset step by the usable A
yCODE
range (i.e. 510):
Eqn. 6
Once the sensitivity
A
y-STEP
has been calculated, the acceleration A
y
can be calculated by the re-using the A
y-STEP
@ A
yCODE
1 value of the offset step and the returned A
yCODE
value with the following transfer function:
Eqn. 7
The pressure, and optional X or Z-axis accelerometer also share the same signal path in the Transducer interface and all the
sensors share the same ADC. Therefore only one of the sensors can be accessed at a given moment.
10.5
Optional Battery Condition Check
The condition of the battery can be periodically checked to determine the battery’s internal impedance, R
BATT
, which is a function
of both temperature and the remaining battery capacity. This can be performed by user supplied software routine and an external
load resistor, R
LOAD
, connected from the PTA0 pin to V
SS
as shown in
(any of the PTA[3:0] can be used for this
purpose).
NOTE
The included accelerometers are designed with a self-test feature. Consult sales/application
support for information regarding the recommended use of the accelerometer self-test
features.
T
T
T
CODE
55
–
=
V
INT
V
INT
V
CODE
1.22
+
=
V
PTAx
V
EXT
Gx
CODE
=
A
y-STEP
A
y-STEP
@ A
yCODE
510
A
y-STEP
@ A
yCODE
1
–
510
=
A
y
A
y-STEP
A
yCODE
A
y-STEP
@ A
yCODE
1
A
y-STEP
–
+
=
Summary of Contents for FXTH870 D Series
Page 86: ...FXTH870xD Sensors 84 Freescale Semiconductor Inc Figure 57 Data Flow For Measurements...
Page 171: ...FXTH870xD Sensors Freescale Semiconductor Inc 169 Figure 128 QFN Case Outline...
Page 172: ...FXTH870xD Sensors 170 Freescale Semiconductor Inc Figure 129 QFN Case Outline...
Page 173: ...FXTH870xD Sensors Freescale Semiconductor Inc 171...
