MAX6954 Maxim, MAX6954 Datasheet - Page 12

• Manufacturer Part Number: MAX6954
• Description: 4-Wire Interfaced / 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan
• Manufacturer: Maxim

4-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
The scan-limit register sets how many 14-segment dig-
its or 16-segment digits or pairs of 7-segment digits are
displayed, from 1 to 8. A bicolor digit is connected as
two monocolor digits. The scan register also limits the
number of keys that can be scanned.
Since the number of scanned digits affects the display
brightness, the scan-limit register should not be used to
blank portions of the display (such as leading-zero sup-
pression). Table 25 shows the scan-limit register format.
Digital control of display brightness is provided and
can be managed in one of two ways: globally or indi-
vidually. Global control adjusts all digits together.
Individual control adjusts the digits separately.
The default method is global brightness control, which
is selected by clearing the global intensity bit (I data bit
D6) in the configuration register. This brightness setting
applies to all display digits. The pulse-width modulator
is then set by the lower nibble of the global intensity
register, address 0x02. The modulator scales the aver-
age segment current in 16 steps from a maximum of
15/16 down to 1/16 of the peak current. The minimum
interdigit blanking time is set to 1/16 of a cycle. When
using bicolor digits, 256 color/brightness combinations
are available.
Individual brightness control is selected by setting the
global intensity bit (I data bit D6) in the configuration
register. The pulse-width modulator is now no longer
set by the lower nibble of the global intensity register,
address 0x02, and the data is ignored. Individual digi-
tal control of display brightness is now provided by a
separate pulse-width modulator setting for each digit.
Each digit is controlled by a nibble of one of the four
intensity registers: intensity10, intensity32, intensity54,
and intensity76 for all display types, plus intensity10a,
intensity32a, intensity54a, and intensity76a for the extra
eight digits possible when 7-segment displays are
used. The data from the relevant register is used for
each digit as it is multiplexed. The modulator scales the
average segment current in 16 steps in exactly the
same way as global intensity adjustment.
Table 26 shows the global intensity register format,
Table 27 shows individual segment intensity registers,
Table 28 is the even individual segment intensity for-
mat, and Table 29 is the odd individual segment inten-
sity format.
The MAX6954 feature five general-purpose input/output
(GPIO) ports: P0 to P4. These ports can be individually
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GPIO and Key Scanning
Scan-Limit Register
Intensity Registers
enabled as logic inputs or open-drain logic outputs.
The GPIO ports are not debounced when configured as
inputs. The ports can be read and the outputs set using
the 4-wire interface.
Some or all of the five ports can be configured to per-
form key scanning of up to 32 keys. Ports P0 to P4 are
renamed Key_A, Key_B, Key_C, Key_D, and IRQ,
respectively, when used for key scanning. The full key-
scanning configuration is shown in Figure 7. Table 30 is
the GPIO data register.
One diode is required per key switch. These diodes
can be common-anode dual diodes in SOT23 pack-
ages, such as the BAW56. Sixteen diodes would be
required for the maximum 32-key configuration.
The MAX6954 can only scan the maximum 32 keys if
the scan-limit register is set to scan the maximum eight
digits. If the MAX6954 is driving fewer digits, then a
maximum of (4 x n) switches can be scanned, where n
is the number of digits set in the scan-limit register. For
example, if the MAX6954 is driving four 14-segment
digits cathode drivers O0 to O3 are used. Only 16 keys
can be scanned in this configuration; the switches
shown connected to O4 through O7 are not read.
If the user wishes to scan fewer than 32 keys, then
fewer scan lines can be configured for key scanning.
The unused Key_x ports are released back to their orig-
inal GPIO functionality. If key scanning is enabled,
regardless of the number of keys being scanned, P4 is
always configured as IRQ (Table 31).
The key-scanning circuit utilizes the LEDs’ common-
cathode driver outputs as the key-scan drivers. O0 to
07 go low for nominally 200µs (with OSC = 4MHz) in
turn as the displays are multiplexed. By varying the
oscillator frequency, the debounce time changes,
though key scanning still functions. Key_x inputs have
internal pullup resistors that allow the key condition to
be tested. The Key_x input is low during the appropri-
ate digit multiplex period when the key is pressed. The
timing diagram of Figure 8 shows the normal situation
where all eight LED cathode drivers are used.
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).