33975 Datasheet, PDF (15/32 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface with Suppressed Wake-Up and 32mA Wetting Current

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
WAKE-UP/INTERRUPT REGISTER
The Wake-Up/Interrupt Register defines the inputs that
are allowed to wake the 33975 from Sleep mode or set the
INT terminal low in Normal mode. Programming the wake-up/
interrupt bit to logic [0] will disable the specific input from
generating an interrupt and will disable the specific input from
waking the IC in Sleep mode (refer to Table 7). Programming
Table 7. Wake-Up /Interrupt Command
the wake-up/interrupt bit to logic [1] will enable the specific
input to generate an interrupt with switch change of state and
will enable the specific input as wake-up. The MCU may
change or update the Wake-Up/Interrupt Register via
software at any time in Normal mode.
Wake-Up/Interrupt Command
Command Bits
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 1 0 X X X X X X X X sp7 sp6 sp5 sp4 sp3 sp2 sp1 sp0
0 0 0 0 0 0 1 1 X X sg1 sg1 sg1 sg1 sg9 sg8 sg7 sg6 sg5 sg4 sg3 sg2 sg1 sg0
3210
WETTING CURRENT REGISTER
The 33975 has two levels of switch-to-ground contact
current, 32 mA and 4.0 mA, and two levels of switch-to-
battery contact current, 16 mA and 2.0 mA (see Figure 10).
The metallic command is used to set the switch contact
current level (refer to Table 8). Programming the metallic bit
to logic [0] will set the switch wetting current to 2.0 mA/4.0
mA. Programming the metallic bit to logic [1] will set the
switch contact wetting current to 16 mA/32 mA. The MCU
may change or update the Wetting Current Register via
software at any time in Normal mode.
Wetting current is designed to provide higher levels of
current during switch closure. The higher level of current is
designed to keep switch contacts from building up oxides that
form on the switch contact surface.
Switch Contact Voltage
32 mA Switch Wetting Current
4.0 mA Switch Sustain Current
20 ms Wetting Current Timer
Figure 10. Contact Wetting and Sustain Current
for Switch-to-Ground Input
Table 8. Metallic Command
Metallic Command
Command Bits
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 1 0 0 X X X X X X X X sp7 sp6 sp5 sp4 sp3 sp2 sp1 sp0
0 0 0 0 0 1 0 1 X X sg1 sg1 sg1 sg1 sg9 sg8 sg7 sg6 sg5 sg4 sg3 sg2 sg1 sg0
3210
WETTING CURRENT TIMER REGISTER
Each switch input has a designated 20 ms timer. The timer
starts when the specific switch input crosses the comparator
threshold (4.0 V). When the 20 ms timer expires, the contact
current is reduced from 16 mA to 2.0 mA for switch-to-battery
inputs and 32 mA to 4.0 mA for switch-to-ground inputs. The
wetting current timer may be disabled for a specific input.
When the timer is disabled, wetting current will continue to
flow through the closed switch contact. With multiple wetting
current timers disabled, power dissipation for the IC must be
considered.
The MCU may change or update the Wetting Current
Timer Register via software at any time in Normal mode. This
allows the MCU to control the amount of time wetting current
is applied to the switch contact. Programming the wetting
current timer bit to logic [0] will disable the wetting current
timer. Programming the wetting current timer bit to logic [1]
will enable the wetting current timer (refer to Table 9).
Analog Integrated Circuit Device Data
Freescale Semiconductor
33975
15