NXP Semiconductors
UM11227
NTM88 family of tire pressure monitor sensors
UM11227 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2020. All rights reserved.
User manual Rev. 6 — 24 April 2020
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aaa-035952
t
INIT
t
MEAS
t
FIN
t
S4WU
t
ADC
MCU Activity
MFO
SEN
SFO
ADC Trigger
ADC Activity
ADC COCO
CLR_SEN
CLR_SEN_SYNC
RUN STOP4 RUN
IDLE IDLE
Figure 45. Automatic measurement protocol
10.19.1.2 SMI direct sensor signal measurement mode
In the direct mode of operation, the sensor signal measurement can be directly controlled
by the user software by managing the SMIC[7] (SMIEN) and SMIC[6] (SCAP) control bits
as shown in Figure 46.
For applications that require accurate measurements, direct mode is not recommended
because MCU clock activity induces ripple on the supply and ground that can increase
the noise and degrade the accuracy of the measurements. Low power direct mode or
automatic mode produce lower noise and offset because the measurements can be
taken when the MCU is in STOP4 mode, where its clock is halted.
The typical sensor measurement software sequence would be to first select the SMI
ADC multiplexer channel, select the SMI transducer channel, and filter. The software
application then sets the SMIEN bit, then delays a period t
AC
, then sets the SCAP
bit. The SMI will then use the SCAP bit being set to synchronize, such that after the
period t
CE
the ADC Trigger will be set. The SCAP bit will be cleared after one full clock
cycle. The ADC will then complete the conversion and the ADC COCO interrupt will be
asserted.
At this point, the application may choose to stop measurements by clearing the SMIEN
bit. Or, if additional measurements are desired, the software application may simply set
the SCAP bit again to repeat the next measurement.
Note that if the software application will change any settings within the SMI, such as the
transducer channel or filter, then the application should again delay by the period t
AC
until
setting the SCAP bit.
Also note that the SMIEN cannot be cleared while the SCAP bit remains set. The
recommended procedure is to poll the SCAP bit until it is clear before clearing the SMIEN
bit. After the SMIEN bit is cleared, the system returns to Standby mode.
Please review the corresponding data sheet for the period noted as t
CE.
The period t
AC
is decided by the software application, and should be set sufficient to
assure useful results.
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