33884 Datasheet, PDF (12/21 Pages) Freescale Semiconductor, Inc – Switch Monitor Interface

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
POWER-UP
On initial power-up all 33884 registers are cleared and the
device enters the Sleep mode. To exit the Sleep mode, a
valid command word is required to be received from the
microprocessor.
SLEEP COMMAND
Sleep mode can be entered by a SPI Sleep command or
asserting the RST pin. In Sleep mode all inputs are Tri–State
and all internal active pull up and pull down currents are
disabled. Sleep mode reduces the current drain to a
quiescent current level of 10 µA and disables the IC. Sleep
mode provides lowest quiescent current for the IC. Exit from
sleep mode requires a valid SPI RUN, TRI–STATE, or
METALLIC command.
RUN COMMAND
The Run command places the IC in one of three operating
modes:
1. This is the normal operating mode of the 33884. In
Normal mode the status of the input switches are
latched on falling edge of CS and data is returned to
the MCU via SPI. All programmed combinations of
source and sink currents, used for sensing purposes,
are always active in this mode. While in the Normal
mode, an interrupt is generated and sent to the
microprocessor whenever an external switch changes
its OPEN or CLOSED state. Prior to a switch closing,
the 33884 sources 0.75 mA of sustain current. When
the voltage at the input crosses the comparator
threshold, 14 mA of current is allowed to flow. The 14
mA wetting current shuts off after a 20 ms timer
expires.
2. The Polling mode reads a switch status periodically,
interrupting the microprocessor only when an external
switch is sensed as being CLOSED. When the 33884
senses all external switches to be OPEN, the Polling
mode of operation continues. When a switch is sensed
CLOSED, an interrupt is sent to the microprocessor,
transferring it’s operational mode to the Normal mode.
The Polling mode provides a reduction in quiescent
current by turning OFF all source and sink currents
during sensed switch OFF periods. The Polling mode
allows reduction of quiescent current by disabling sink
and source currents during switch OFF periods.
3. The Polling + INT Timer mode of operation is similar to
the Polling mode above, except with the addition of an
interrupt being sent to the microprocessor if a switch is
sensed CLOSED, or upon the internal interrupt timer
timing out. An interrupt is always ultimately sent to the
microprocessor in this mode. The microprocessor can
be programmed to read, or ignore the reported switch
status while receiving the interrupt. If a switch is
sensed CLOSED, operation automatically reverts to
the Normal mode. If all switches are sensed OPEN,
and the wake up timer (INT Timer) times out, the 33884
continues to operate in the Polling + INT Timer mode.
The wake up timer duration may be set much longer
than the polling time.
The command also programs the SP1 to SP4 sense inputs
(switch-to-battery logic [1] or switch-to-ground logic[0]).
Please refer to Table 17.
TRI-STATE COMMAND
A Tri-State command places all switch inputs into Tri-State
position. All comparators on input are disabled in this mode.
The device will return logic [0] for the switch status.
SPI PROGRAMMING
The 33884 uses the SPI in full duplex synchronous slave
mode for communication with the microprocessor. The 33884
is programmed via a 16-bit word command from the MCU.
The word is sent to the device with the MSB first. The
command word sent to the 33884 sets the mode of operation
in the device. Returning data received from the 33884 is the
status of the sensed input switch on the falling edge of CS.
Sixteen clock periods are required for each transmission to
be valid. After the 16 clocks, CS is returned to the inactive
state (logic [1]), command words are no longer accepted into
SI, and the SO pin is Tri-Stated. The response to a SPI
command returns status based on previous command word.
This previous command could be a hardware reset as well as
any of the other commands discussed in this section.
33884
12
Analog Integrated Circuit Device Data
Freescale Semiconductor