33975 Datasheet, PDF (20/32 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface with Suppressed Wake-Up and 32mA Wetting Current

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
critical to understand INT will not be cleared on the rising
edge of CS if a switch closure occurs while CS is low. The
maximum duration a switch state change can exist without
acknowledgement depends on the software response time to
the interrupt. Figure 6, page 10, shows the interaction
between changing input states and the INT and CS terminals.
If desired the user may disable interrupts (wake up/
interrupt command) from the 33975 device and read the
switch states on a periodic basis. Switch activation and
deactivation faster than the MCU read rate will not be
acknowledged.
The 33975 device will exit the Normal mode and enter the
Sleep mode only with a valid sleep command.
SLEEP MODE
Sleep mode is used to reduce system quiescent currents.
Sleep mode may be entered only by sending the sleep
command. All register settings programmed in Normal mode
will be maintained in Sleep mode.
The 33975 will exit Sleep mode and enter Normal mode
when any of the following events occur:
• Input Switch Change of State (when enabled)
Table 18. Sleep Command
• Interrupt Timer Expire
• Falling Edge of WAKE
• Falling Edge of INT (with VDD = 5.0 V and WAKE at
Logic [1])
• Falling Edge of CS (with VDD = 5.0 V)
• Power-ON Reset (POR)
The VDD supply may be removed from the device during
Sleep mode. However removing VDD from the device in
Sleep mode will disable a wake-up from falling edge of INT
and CS.
Note In cases where CS is used to wake the device, the
first SO data message is not valid.
The sleep command contains settings for two
programmable timers for Sleep mode, the interrupt timer and
the scan timer, as shown in Table 18 The interrupt timer is
used as a periodic wake-up timer. When the timer expires, an
interrupt is generated and the device enters Normal mode.
Note The interrupt timer in the 33975 device may be
disabled by programming the interrupt bits to logic [1 1 1].
Table 19 shows the programmable settings of the Interrupt
timer.
Sleep 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 1 1 0 0XXXXXXXXXX
Table 19. Interrupt Timer
Bits 543
Interrupt Period
000
32 ms
001
64 ms
010
128 ms
011
256 ms
100
512 ms
101
1.024 s
110
2.048 s
111
No interrupt wake-up
The scan timer sets the polling period between input
switch reads in Sleep mode. The period is set in the sleep
command and may be set to 000 (no period) to 111 (64 ms).
In Sleep mode when the scan timer expires, inputs will
behave as programmed prior to sleep command. The 33975
will wake up for approximately 125 µs and read the switch
inputs. At the end of the 125 µs, the input switch states are
compared with the switch state prior to sleep command.
33975
20
When switch state changes are detected, an interrupt (when
enabled; refer to wake-up/interrupt command description on
page 15) is generated and the device enters Normal mode.
Without switch state changes, the 33975 will reset the scan
timer, inputs become tri-state, and the Sleep mode continues
until the scan timer expires again.
Table 20 shows the programmable settings of the Scan
timer.
Table 20. Scan Timer
Bits 210
Scan Period
000
No Scan
001
1.0 ms
010
2.0 ms
011
4.0 ms
100
8.0 ms
101
16 ms
110
32 ms
111
64 ms
Note The interrupt and scan timers are disabled in the
Normal mode.
Analog Integrated Circuit Device Data
Freescale Semiconductor