sx8639 Semtech Corporation, sx8639 Datasheet - Page 12

• Manufacturer Part Number: sx8639
• Description: Low Power, Capacitive Button And Wheel Touch Controller 8 Sensors With Enhanced Led Drivers And Proximity Sensing
• Manufacturer: Semtech Corporation

SX8639
Low Power, Capacitive Button and Wheel Touch Controller
(8 sensors) with Enhanced LED Drivers and Proximity Sensing
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
3.3 Scan Period
The basic operation Scan period of the SX8639 sensing interface can be split into three periods over time.
In the first period (Sensing) the SX8639 is sensing all enabled CAP inputs, from CAP0 towards CAP7.
In the second period (Processing) the SX8639 processes the sensor data, verifies and updates the GPIO and I2C
status registers.
In the third period (Timer) the SX8639 is set in a low power mode and waits until a new cycle starts.
Figure 7 shows the different SX8639 periods over time.
Scan Period
Figure 7
The scan period determines the minimum reaction time of the SX8639. The scan period can be configured by the
host from 15ms to values larger than a second.
The reaction time is defined as the interval between a touch on the sensor and the moment that the SX8639
generates the interrupt on the INTB pin. The shorter the scan period the faster the reaction time will be.
Very low power consumption can be obtained by setting very long scan periods with the expense of having longer
reaction times.
Important: All external events like GPIO, I2C and INTB are updated in the processing period, so once every scan
period. If e.g. a GPI would change state directly after the processing period then this will be reported with a delay
of one scan period later in time.
3.4 Operation modes
The SX8639 has 3 operation modes. The main difference is found in the reaction time (corresponding to the scan
period) and power consumption.
Active mode offers fast scan periods. The typical reaction time is 30ms. All enabled sensors are scanned and
information data is processed within this interval.
Doze mode increases the scan period time which increases the reaction time to 195ms typical and at the same
time reduces the operating current.
Sleep mode turns the SX8639 OFF, except for the I2C and GPI peripheral, minimizing operating current while
maintaining the power supplies. In Sleep mode the SX8639 does not do any sensor scanning.
The user can specify other scan periods for the Active and Doze mode and decide for other compromises
between reaction time and power consumption.
In most applications the reaction time needs to be fast when fingers are present, but can be slow when no person
uses the application. In case the SX8639 is not used for a specific time it can go from Active mode into Doze
mode and power will be saved. This time-out is determined by the Passive Timer which can be configured by the
user or turned OFF if not required.
Revision 7_8, September 10
© 2010 Semtech Corp.
www.semtech.com
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