33975 Datasheet, PDF (25/32 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface with Suppressed Wake-Up and 32mA Wetting Current

TYPICAL APPLICATIONS
OPERATIONAL MODES
Multiple 33975 devices may be used in a module system.
SPI control may be done in parallel or serial. However when
parallel mode is used, each device is addressed
independently (refer to MCU Interface Description, page 13).
Therefore when sending the sleep command, one device will
enter sleep before the other. For multiple devices in a system,
it is recommended that the devices are controlled in serial (S0
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 terminals should be connected to the enable
terminal on the power supply. The INT terminals may be
connected to one interrupt terminal 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 terminal will be logic [1]. If either device wakes up,
the WAKE terminal 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 33975 IC has an internal 5.0 V supply from the VPWR
terminal. A POR circuit monitors the internal 5.0 V supply. In
the event of transients on the VPWR terminal, an internal
reset may occur. Upon reset the 33975 will enter Normal
mode with the internal registers as defined in Table 15,
page 18. Therefore it is recommended that the MCU
periodically update all registers internal to the IC.
USING THE WAKE FEATURE
The 33975 provides a WAKE output and wake-up input
designed to control an enable terminal on system power
supply. While in the Normal mode, the WAKE output is low,
enabling the power supply. In the Sleep mode, the WAKE
terminal is high, disabling the power supply. The WAKE
terminal 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 26).
When the WAKE output is not used the terminal should be
pulled up to the VDD supply through a resistor as shown in
Figure 15, page 26).
During the Sleep mode, a switch closure will set the WAKE
terminal low, causing the 33975 to enter the Normal mode.
The power supply will then be activated, supplying power to
the VDD terminal and the microprocessor and the 33975. 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
33975
25