33975 Datasheet, PDF (11/32 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface with Suppressed Wake-Up and 32mA Wetting Current

FUNCTIONAL DESCRIPTIONS
INTRODUCTION
FUNCTIONAL DESCRIPTIONS
INTRODUCTION
The 33975 device is an integrated circuit designed to
provide systems with ultra-low quiescent sleep/wake-up
modes and a robust interface between switch contacts and a
microprocessor. The 33975 replaces many of the discrete
components required when interfacing to microprocessor-
based systems while providing switch ground offset
protection, contact wetting current, and system wake-up.
The 33975 features 8-programmable switch-to-ground or
switch-to-battery inputs and 14 switch-to-ground inputs. All
switch inputs may be read as analog inputs through the
analog multiplexer (AMUX). Other features include a
programmable wake-up timer, programmable interrupt timer,
programmable wake-up/interrupt bits, and programmable
wetting current settings.
This device is designed primarily for automotive
applications but may be used in a variety of other applications
such as computer, telecommunications, and industrial
controls.
FUNCTIONAL TERMINAL DESCRIPTION
CHIP SELECT (CS)
The system MCU selects the 33975 to receive
communication using the chip select (CS) terminal. With the
CS in a logic low state, command words may be sent to the
33975 via the serial input (SI) terminal, and switch status
information can be received by the MCU via the serial output
(SO) terminal. The falling edge of CS enables the SO output,
latches the state of the INT terminal, and the state of the
external switch inputs.
Rising edge of the CS initiates the following operation:
1. Disables the SO driver (high impedance)
1. INT terminal is reset to logic [1], except when additional
switch changes occur during CS low (see Figure 6,
page 10).
1. Activates the received command word, allowing the
33975 to act upon new data from switch inputs.
To avoid any spurious data, it is essential the high-to-low
and low-to-high transitions of the CS signal occur only when
SCLK is in a logic low state. Internal to the 33975 device is an
active pull-up to VDD on CS.
In Sleep mode the negative edge of CS (VDD applied) will
wake up the 33975 device. Data received from the device
during CS wake-up may not be accurate.
SERIAL CLOCK (SCLK)
The system clock (SCLK) terminal clocks the internal shift
register of the 33975. The SI data is latched into the input
shift register on the falling edge of SCLK signal. The SO
terminal shifts the switch status bits out on the rising edge of
SCLK. The SO data is available for the MCU to read on the
falling edge of SCLK. False clocking of the shift register must
be avoided to ensure validity of data. It is essential the SCLK
terminal be in a logic low state whenever CS makes any
transition. For this reason, it is recommended, though not
necessary, that the SCLK terminal is commanded to a low
logic state as long as the device is not accessed and CS is in
a logic high state. When the CS is in a logic high state, any
signal on the SCLK and SI terminals will be ignored and the
SO terminal is tri-state.
SERIAL INPUT (SI)
The SI terminal is used for serial instruction data input. SI
information is latched into the input register on the falling
edge of SCLK. A logic high state present on SI will program
a one in the command word on the rising edge of the CS
signal. To program a complete word, 24 bits of information
must be entered into the device.
SERIAL OUTPUT (SO)
The SO terminal is the output from the shift register. The
SO terminal remains tri-stated until the CS terminal
transitions to a logic low state. All open switches are reported
as zero, all closed switches are reported as one. The
negative transition of CS enables the SO driver.
The first positive transition of SCLK will make the status
data bit 24 available on the SO terminal. Each successive
positive clock will make the next status data bit available for
the MCU to read on the falling edge of SCLK. The SI/SO
shifting of the data follows a first-in-first-out protocol, with
both input and output words transferring the most significant
bit (MSB) first.
INTERRUPT OUTPUT (INT)
The INT terminal is an interrupt output from the 33975
device. The INT terminal is an open-drain output with an
internal pull-up to VDD. In Normal mode, a switch state
change will trigger the INT terminal (when enabled). The INT
terminal is latched on the falling edge of CS. and cleared on
the rising edge of CS. The INT terminal will not clear with
rising edge of CS if a switch contact change has occurred
while CS was low.
In a multiple 33975 device system with WAKE high and
VDD on (Sleep mode), the falling edge of INT will place all
33975s in Normal mode.
Analog Integrated Circuit Device Data
Freescale Semiconductor
33975
11