The timing in Figure 8 loops over time, with 32 keys
experiencing a full key-scanning debounce over typi-
cally 25.6ms. Four keys are sampled every 1.6ms, or
every multiplex cycle. If at least one key that was not
previously pressed is found to have been pressed dur-
ing both sampling periods, then that key press is
debounced, and an interrupt is issued. The key-scan
circuit detects any combination of keys being pressed
during each debounce cycle (n-key rollover).
Port Configuration Register
The port configuration register selects how the five port
pins are used. The port configuration register format is
described in Table 32.
Key Mask Registers
The Key_A Mask, Key_B Mask, Key_C Mask, and
Key_D Mask write-only registers (Table 33) configure
the key-scanning circuit to cause an interrupt only when
selected (masked) keys have been debounced. Each
bit in the register corresponds to one key switch. The bit
is clear to disable interrupt for the switch, and set to
enable interrupt. Keys are always scanned (if enabled
through the port configuration register), regardless of
the setting of these interrupt bits, and the key status is
stored in the appropriate Key_x pressed register.
Key Debounced Registers
The Key_A debounced, Key_B debounced, Key_C
debounced, and Key_D debounced read-only registers
(Table 34) show which keys have been detected as
debounced by the key-scanning circuit.
Each bit in the register corresponds to one key switch.
The bit is set if the switch has been correctly
debounced since the register was read last. Reading a
debounced register clears that register (after the data
has been read) so that future keys pressed can be
identified. If the debounced registers are not read, the
key-scan data accumulates. However, as there is no
FIFO in the MAX6954, the user is not able to determine
key order, or whether a key has been pressed more
than once, unless the debounced key status registers
are read after each interrupt, and before the next key-
scan cycle.
Reading any of the four debounced registers clears the
IRQ output. If a key is pressed and held down, the key is
reported as debounced (and IRQ issued) only once.
The key must be detected as released by the key-scan-
ning circuit, before it debounces again. If the
debounced registers are being read in response to the
IRQ being asserted, then the user should generally read
all four registers to ensure that all the keys that were
detected by the key-scanning circuit are discovered.
Key Pressed Registers
The Key_A pressed, Key_B pressed, Key_C pressed,
and Key_D pressed read-only registers (Table 35)
show which keys have been detected as pressed by
the key-scanning circuit during the last test.
Each bit in the register corresponds to one key switch.
The bit is set if the switch has been detected as
pressed by the key-scanning circuit during the last test.
MAX6954
4-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
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