MC33972_08 Datasheet, PDF (20/32 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface with Suppressed Wake-Up

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
Table 19. Interrupt Timer
Bits 543
Interrupt Period
000
32ms
001
64ms
010
128ms
011
256ms
100
512ms
101
1.024s
110
2.048s
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 (64ms).
In Sleep Mode when the scan timer expires, inputs will
behave as programmed prior to sleep command. The 33972
will wake up for approximately 125μs and read the switch
inputs. At the end of the 125μs, the input switch states are
compared with the switch state prior to sleep command.
When switch state changes are detected, an interrupt is
generated (when enabled; refer to wake-up / interrupt
command description on page 15), and the device enters
Normal Mode. Without switch state changes, the 33972 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
000
001
010
011
100
101
110
111
Scan Period
No Scan
1.0ms
2.0ms
4.0ms
8.0ms
16ms
32ms
64ms
Note The interrupt and scan timers are disabled in the
Normal Mode.
Figure 5, page 9, is a graphical description of how the
33972 device exits Sleep Mode and enters Normal Mode.
Notice that the device will exit Sleep Mode when the interrupt
timer expires or when a switch change of state occurs. The
falling edge of INT triggers the MCU to wake from Sleep
state. Figure 12 illustrates the current consumed during
Sleep Mode. During the 125μs, the device is fully active and
switch states are read. The quiescent current is calculated by
integrating the normal running current over scan period plus
approximately 60μA.
I^I==^VV^/R/R^oro⁰r^.20⁷⁰.2^V7^/10⁰V⁰/^o1^h0^m0⁼Ω^2.=^7m2^A.7mA
Ion^I_u1npt₂u^p_o5tf^us_μ3a^ts₂^sct_omi^av_ucsettfⁱo_ovr^ef1₃2^fo25_mrus_s
II==VV/R/Roro6rmV/100ohm = 60 uA
6.0mV/100Ω = 60μA
Figure 12. Sleep Current Waveform
TEMPERATURE MONITOR
With multiple switch inputs closed and the device
programmed with the wetting current timers disabled,
considerable power will be dissipated by the IC. For this
reason, temperature monitoring has been implemented. The
temperature monitor is active in the Normal Mode only. When
the IC temperature is above the thermal limit, the temperature
monitor will do all of the following:
• Generate an interrupt.
• Force all 16mA pull-up and pull-down current sources to
revert to 2.0mA current sources.
• Maintain the 2.0mA current source and all other
functionality.
• Set the thermal flag bit in the SPI output register.
The thermal flag bit in the SPI word will be cleared on rising
edge of CS provided the die temperature has cooled below
the thermal limit. When die temperature has cooled below
thermal limit, the device will resume previously programmed
settings.
33972
20
Analog Integrated Circuit Device Data
Freescale Semiconductor