28.3.3 Sensor Timing
For each channel in the scan sequence, the LESENSE interface goes through three phases: idle, excite, and measure. The durations
of the excite and measure phases are configured in the CHx_TIMING registers. The excite phase duration can be configured to be
either a number of AUXHFRCO cycles or a number of LFACLK
LESENSE
cycles, depending on which one is selected by the EXCLK bit in
the CHx_INTERACT register. LESENSE includes two timers: A low frequency timer, running on LFACLK
LESENSE
, and a high frequency
timer, running on AUXHFRCO. The low frequency or high frequency timers can be prescaled by configuring LFPRESC or AUXPRESC,
respectively, in the TIMCTRL register. The duration of the measure phase is programmed via MEASUREDLY and SAMPLEDLY in the
CHx_TIMING registers. The output of the ACMP will be ignored for MEASUREDLY EXCLK cycles after start of the sensor measure-
ment. Sampling of the sensor will happen after SAMPLEDLY LFACLK
LESENSE
, or AUXHFRCO cycles, depending on the configuration
of the SAMPLECLK in the CHx_INTERACT register. The configurable measure- and sample delays enables LESENSE to easily define
exact time windows for sensor measurements. A start delay can be inserted before sensor measurement begin by configuring
STARTDLY in TIMCTRL. This delay can be used to ensure that the VDAC conversion is done and voltages have stabilized before the
sensor measurement begins. The AUXHFRCO startup can be delayed until the system enters the excite phase, by configuring AUX-
STARTUP in TIMCTRL to ONDEMAND. This will reduce the time the AUXHFRCO is enabled and reduce power consumption, with the
tradeoff that that the starting point for high frequency timing will also be delayed the same amount as the AUXHFRCO startup time.
Figure 28.4 Timing Diagram, AUXHFRCO Based Timing on page 935
depicts a sensor sequence with AUXHFRCO based timing (EX-
TIME=5, MEASUREDLY=7, SAMPLEDLY=13), while
Figure 28.5 Timing Diagram, LFACLK Based Timing on page 936
quence with LFACLK
LESENSE
based timing (EXTIME=1, MEASUREDLY=1, SAMPLEDLY=2).
EXCITE
SAMPLE
LFACLK
LESENSE
Idle phase
Excite phase
Idle phase
SAMPLEDLY
Measure phase
START
AUXHFRCO
INIT
STARTDLY
MEASUREDLY
DAC refresh start
Synchronization
delay
EXTIME
Figure 28.4. Timing Diagram, AUXHFRCO Based Timing
Reference Manual
LESENSE - Low Energy Sensor Interface
silabs.com
| Building a more connected world.
Rev. 1.1 | 935