MAX6954
4-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
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A logic high selects hexadecimal decoding, while a
logic low bypasses the decoder. When direct control is
selected, the data bits D7 to D0 correspond to the seg-
ment lines of the MAX6954. Write x0010000 to blank all
segments in hexadecimal decode mode.
Display Blink Mode
The display blinking facility, when enabled, makes the
driver flip automatically between displaying the digit
register data in planes P0 and P1. If the digit register
data for any digit is different in the two planes, then that
digit appears to flip between two characters. To make a
character appear to blink on or off, write the character
to one plane, and use the blank character (0x20) for the
other plane. Once blinking has been configured, it con-
tinues automatically without further intervention.
Blink Speed
The blink speed is determined by the frequency of the
multiplex clock, OSC, and by the setting of the Blink
Rate Selection Bit B (Table 18) in the configuration reg-
ister. The Blink Rate Selection Bit B sets either fast or
slow blink speed for the whole display.
Initial Power-Up
On initial power-up, all control registers are reset, the
display is blanked, intensities are set to minimum, and
shutdown is enabled (Table 15).
Configuration Register
The configuration register is used to enter and exit shut-
down, select the blink rate, globally enable and disable
the blink function, globally clear the digit data, select
between global or digit-by-digit control of intensity, and
reset the blink timing (Tables 16–19 and 21–24).
The configuration register contains 7 bits:
• S bit selects shutdown or normal operation
(read/write).
• B bit selects the blink rate (read/write).
• E bit globally enables or disables the blink function
(read/write).
• T bit resets the blink timing (data is not stored—tran-
sient bit).
• R bit globally clears the digit data for both planes P0
and P1 for ALL digits (data is not stored—transient
bit).
• I bit selects between global or digit-by-digit control
of intensity (read/write).
• P bit returns the current phase of the blink timing
(read only—a write to this bit is ignored).
Character Generator Font Mapping
The font is composed of 104 characters in ROM. The
lower 7 bits of the 8-bit digit register represent the char-
acter selection. The most significant bit, shown as x in
the ROM map of Tables 7 and 8, is 1 to light the DP
segment and zero to leave the DP segment unlit.
The character map follows the standard ASCII font for
96 characters in the x0101000 through x1111111
range. The first 16 characters of the 16-segment ROM
map cover 7-segment displays. These 16 characters
are numeric 0 to 9 and characters A to F (i.e., the hexa-
decimal set).
Multiplex Clock and Blink Timing
The OSC pin can be fitted with capacitor C
SET
to GND
to use the internal RC multiplex oscillator, or driven by
an external clock to set the multiplex clock frequency
and blink rate. The multiplex clock frequency deter-
mines the frequency that the complete display is updat-
ed. With OSC at 4MHz, each display digit is enabled for
200µs.
The internal RC oscillator uses an external resistor,
R
SET
, and an external capacitor, C
SET
, to set the oscil-
lator frequency. The suggested values of R
SET
(56kΩ)
and C
SET
(22pF) set the oscillator at 4MHz, which
makes the blink frequency 0.5Hz or 1Hz.
The external clock is not required to have a 50:50 duty
cycle, but the minimum time between transitions must
be 50ns or greater and the maximum time between
transitions must be 750ns.
The on-chip oscillator may be accurate enough for
applications using a single device. If an exact blink rate
is required, use an external clock ranging between
1MHz and 8MHz to drive OSC. The OSC inputs of multi-
ple MAX6954s can be tied together to a common exter-
nal clock to make the devices blink at the same rate.
The relative blink phasing of multiple MAX6954s can be
synchronized by setting the T bit in the control register
for all the devices in quick succession. If the serial inter-
faces of multiple MAX6954s are daisy-chained by con-
necting the DOUT of one device to the DIN of the next,
then synchronization is achieved automatically by
updating the configuration register for all devices simul-
taneously. Figure 6 is the multiplex timing diagram.
OSC_OUT Output
The OSC_OUT output is a buffered copy of either the
internal oscillator clock or the clock driven into the OSC
pin if the external clock has been selected. The feature
is useful if the internal oscillator is used, and the user
wishes to synchronize other MAX6954s to the same
blink frequency. The oscillator is disabled while the
MAX6954 is in shutdown.
