33993 Datasheet, PDF (23/28 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface

TYPICAL APPLICATIONS
INTRODUCTION
from first device is connected to SI of second device). With
two devices, 48 clock pulses are required to shift data in.
When the WAKE feature is used to enable the power supply,
both WAKE pins should be connected to the enable pin on
the power supply. The INT pins may be connected to one
interrupt pin on the MCU or may have their own dedicated
interrupt to the MCU.
The transition from Normal to Sleep mode is done by
sending the sleep command. With the devices connected in
serial and the sleep command sent, both will enter Sleep
mode on the rising edge of CS. When Sleep mode is entered,
the WAKE pin will be logic [1]. If either device wakes up, the
WAKE pin will transition low, waking the other device.
A condition exists where the MCU is sending the sleep
command (CS logic [0]) and a switch input changes state.
With this event the device that detects this input will not
transition to Sleep mode, while the second device will enter
Sleep mode. In this case two switch status commands must
be sent to receive accurate switch status data. The first
switch status command will wake the device in Sleep mode.
Switch status data may not be valid from the first switch
status command because of the time required for the input
voltage to rise above the 4.0 V input comparator threshold.
This time is dependant on the impedance of SGn or SPn
node. The second switch status command will provide
accurate switch status information. It is recommended that
software wait 10 ms to 20 ms between the two switch status
commands, allowing time for switch input voltages to
stabilize. With all switch states acknowledged by the MCU,
the sleep sequence may be initiated. All parameters for Sleep
mode should be updated prior to sending the sleep
command.
The 33993 IC has an internal 5.0 V supply from V_PWR pin.
A POR circuit monitors the internal 5.0 V supply. In the event
of transients on the V_PWR pin, an internal reset may occur.
Upon reset the 33993 will enter Normal mode with the
internal registers as defined in Table 14, page 16. Therefore
it is recommended that the MCU periodically update all
registers internal to the IC.
USING THE WAKE FEATURE
The 33993 provides a WAKE output and wake-up input
designed to control an enable pin on system power supply.
While in the Normal mode, the WAKE output is low, enabling
the power supply. In the Sleep mode, the WAKE pin is high,
disabling the power supply. The WAKE pin has a passive
pull-up to the internal 5.0 V supply but may be pulled up
through a resistor to V_PWR supply (see Figure 16, page 24)
When the WAKE output is not used the pin should be
pulled up to the V_DD supply through a resistor as shown in
Figure 15, page 24.
During the Sleep mode, a switch closure will set the WAKE
pin low, causing the 33993 to enter the Normal mode. The
power supply will then be activated, supplying power to the
V_DD pin and the microprocessor and the 33993. The
microprocessor can determine the source of the wake-up by
reading the interrupt flag.
COST AND FLEXIBILITY
Systems requiring a significant number of switch
interfaces have many discrete components. Discrete
components on standard PWB consume board space and
must be checked for solder joint integrity. An integrated
approach reduces solder joints, consumes less board space,
and offers wider operating voltage, analog interface
capability, and greater interfacing flexibility.
Analog Integrated Circuit Device Data
Freescale Semiconductor
33993
23