33993 Datasheet, PDF (14/28 Pages) Freescale Semiconductor, Inc – Multiple Switch Detection Interface

FUNCTIONAL DEVICE OPERATION
LOGIC COMMANDS AND REGISTERS
Table 7. 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. 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 (refer to Table 8).
Table 8. 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 sg1 sg1 sg1 sg1 sg9 sg8 sg7 sg6 sg5 sg4 sg3 sg2 sg1 sg0
3210
TRI-STATE REGISTER
The tri-state command is use to set the SPn or SGn input
node as high impedance (refer to Table 9). By setting the
Tri-State Register bit to logic [1], the input will be high
impedance regardless of the metallic command setting. The
Table 9. Tri-State Command
comparator on each input remains active. This command
allows the use of each input as a comparator with a 4.0 V
threshold. The MCU may change or update the Tri-State
Register via software at any time in Normal mode.
Tri-State 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 1 0 0 1 X X X X X X X X sp7 sp6 sp5 sp4 sp3 sp2 sp1 sp0
0 0 0 0 1 0 1 0 X X sg1 sg1 sg1 sg1 sg9 sg8 sg7 sg6 sg5 sg4 sg3 sg2 sg1 sg0
3210
ANALOG SELECT REGISTER
The analog voltage on switch inputs may be read by the
MCU using the analog command (refer to Table 10). Internal
to the IC is a 22-to-1 analog multiplexer. The voltage present
on the selected input pin is buffered and made available on
the AMUX output pin. The AMUX output pin is clamped to a
maximum 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]. When selecting a channel to be read as
analog, the user must also set the desired current (16 mA,
2.0 mA, or high impedance). Setting bit 6 and bit 5 to 0,0
selects the input as high impedance. Setting bit 6 and bit 5 to
0,1 selects 2.0 mA, and 1,0 selects 16 mA. Setting bit 6 and
bit 5 to 1,1 in the Analog Select Register is not allowed and
will place the input as an analog input with high impedance.
Analog currents set by the analog command are pull-up
currents for all SGn and SPn inputs (refer to Table 10). The
analog command does not allow pull-down currents on the
SPn inputs. Setting the current to 16 mA or 2.0 mA may be
useful for reading sensor inputs. Further information is
provided in the Typical Applications section of this datasheet
beginning on page 20. The MCU may change or update the
33993
14
Analog Integrated Circuit Device Data
Freescale Semiconductor