33972_09 Datasheet, PDF (15/32 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface with Suppressed Wake-up

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
Table 7. Wake-up / Interrupt Command
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 sg13 sg12 sg11 sg10 sg9 sg8 sg7 sg6 sg5 sg4 sg3 sg2 sg1 sg0
WETTING CURRENT REGISTER
The 33972 has two levels of switch contact current, 16 and
2.0 mA (see Figure 11). The metallic command is used to set
the switch contact current level (Table 8). Programming the
metallic bit to logic [0] will set the switch wetting current to
2.0 mA. Programming the metallic bit to logic [1] will set the
switch contact wetting current to 16 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
16 mA Switch Wetting Current
2.0 mA Switch Sustain Current
20 ms Wetting Current Timer
Figure 11. Contact Wetting and Sustain Current
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 sg13 sg12 sg11 sg10 sg9 sg8 sg7 sg6 sg5 sg4 sg3 sg2 sg1 sg0
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 to 2.0 mA. The wetting current
timer may be disabled for a specific input. When the timer is
disabled, 16 mA of 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 (Table 9).
Table 9. Wetting Current Timer Enable Command
Wetting Current Timer Commands
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 1 1 X X X X X X X X sp7 sp6 sp5 sp4 sp3 sp2 sp1 sp0
0 0 0 0 1 0 0 0 X X sg13 sg12 sg11 sg10 sg9 sg8 sg7 sg6 sg5 sg4 sg3 sg2 sg1 sg0
Analog Integrated Circuit Device Data
Freescale Semiconductor
33972
15