33975 datasheet(26/32 Pages) FREESCALE

26 / 32 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

33975

TYPICAL APPLICATIONS

OPERATIONAL MODES

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 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

the software wait 10 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

pin. A POR circuit monitors the internal 5.0 V supply. In the

event of transients on the VPWR pin, an internal reset may

occur. Upon reset the 33975 will enter Normal mode with the

internal registers as defined in Table , page 20. 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 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 VPWR supply (see Figure 17, page 27).

When the WAKE output is not used the pin should be pulled

up to the VDD supply through a resistor, as shown in

Figure 16, page 27).

During the Sleep mode, a switch closure will set the WAKE

pin low, causing the 33975 to enter the Normal mode. The

power supply will then be activated, supplying power to the

VDD pin 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.

33975 Datasheet(PDF) 26 Page - Freescale Semiconductor, Inc

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