33993 Datasheet, PDF (20/28 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface

TYPICAL APPLICATIONS
INTRODUCTION
TYPICAL APPLICATIONS
INTRODUCTION
The 33993’s primary function is the detection of open or
closed switch contacts. However, there are many features
that allow the device to be used in a variety of applications.
The following is a list of applications to consider for the IC:
• Sensor Power Supply
• Switch Monitor for Metallic or Elastomeric Switches
• Analog Sensor Inputs (Ratiometric)
• Power MOSFET/LED Driver and Monitor
• Multiple 33993 Devices in a Module System
The following paragraphs describe the applications in
detail.
SENSOR POWER SUPPLY
Each input may be used to supply current to sensors
external to a module. Many sensors such as Hall effect,
pressure sensors, and temperature sensors require a supply
voltage to power the sensor and provide an open collector or
analog output. Figure 12 shows how the 33993 may be used
to supply power and interface to these types of sensors. In an
application where the input makes continuous transitions,
consider using the wake-up / interrupt command to disable
the interrupt for the particular input.
VBAT
VBAT
16 mA
33993
SP0
SP1
V_PWR
V_DD
V_DD
SP7
WAKE
SG0
SG1
V_PWR V_PWR
SI
SCLK
CS
16
mA
SG12
2.0 SO
mA _INT
V_PWR V_PWR
Hall-Effect
Sensor
16
mA
2.0
mA
Reg
SG13
X
2.5 kΩ
2.5 kΩ
MCU
MOSI
SCLK
CS
MISO
INT
IOC[7:0]
Input Capture
Timer Port
Figure 12. Sensor Power Supply
METALLIC / ELASTOMERIC SWITCH
Metallic switch contacts often develop higher contact
resistance over time owing to contact corrosion. The
corrosion is induced by humidity, salt, and other elements
that exist in the environment. For this reason the 33993
provides two settings for contacts. When programmed for
metallic switches, the device provides higher wetting current
to keep switch contacts free of oxides. The higher current
occurs for the first 20 ms of switch closure. Where longer
duration of wetting current is desired, the user may send the
wetting current timer command and disable the timer. Wetting
current will be continuous to the closed switch. After the time
period set by the MCU, the wetting current timer command
may be sent again to enable the timer. The user must
consider power dissipation on the device when disabling the
timer. (Refer to the paragraph entitled Temperature Monitor,
page 18.)
To increase the amount of wetting current for a switch
contact, the user has two options. Higher wetting current to a
switch may be achieved by paralleling SGn or SPn inputs.
This will increase wetting current by 16 mA for each input
added to the switch contact. The second option is to simply
add an external resistor pull-up to the V_PWR supply for switch-
to-ground inputs or a resistor to ground for a switch-to-battery
input. Adding an external resistor has no effect on the
operation of the device.
Elastomeric switch contacts are made of carbon and have
a high contact resistance. Resistance of 1.0 kΩ is common.
In applications with elastomeric switches, the pull-up and
pull-down currents must be reduced to prevent excessive
power dissipation at the contact. Programming for a lower
current settings is provided in the Functional Device
Operation Section beginning on page 11 under Table 7,
Metallic Command.
ANALOG SENSOR INPUTS (RATIOMETRIC)
The 33993 features a 22-to-1 analog multiplexer. Setting
the binary code for a specific input in the analog command
allows the microcontroller to perform analog to digital
conversion on any of the 22 inputs. On rising edge of CS the
multiplexer connects a requested input to the AMUX pin. The
AMUX pin is clamped to max of V_DD volts regardless of the
higher voltages present on the input pin. After an input has
been selected as the analog, the corresponding bit in the next
SO data stream will be logic [0].
The input pin, when selected as analog, may be
configured as analog with high impedance, analog with
2.0 mA pull-up, or analog with 16 mA pull-up. Figure 13,
page 21, shows how the 33993 may be used to provide a
ratiometric reading of variable resistive input.
33993
20
Analog Integrated Circuit Device Data
Freescale Semiconductor