MAX6957 MAXIM [Maxim Integrated Products], MAX6957 Datasheet - Page 10

• Manufacturer Part Number: MAX6957
• Description: 4-Wire-Interfaced, 2.5V to 5.5V, 20-Port and 28-Port LED Display Driver and I/O Expander
• Manufacturer: MAXIM [Maxim Integrated Products]

4) Take CS high (while SCLK is still high after clocking
5) Take SCLK low.
6) Issue another read or write command (which can
On initial power-up, all control registers are reset, cur-
rent registers are set to minimum value, and the
MAX6957 enters shutdown mode (Table 4).
LED segment drive current can be set either globally or
individually. Global control simplifies the operation
when all LEDs are set to the same current level,
because writing one register, the Global Current regis-
ter, sets the current for all ports configured as LED seg-
ment drivers. It is also possible to individually control
the current drive of each LED segment driver.
Individual/global brightness control is selected by set-
ting the configuration register I bit (Table7). The global
current register (0x02) data are then ignored, and seg-
ment currents are set using register addresses 0x12
through 0x1F (Tables 10, 11, and 12). Each segment is
controlled by a nibble of one of the 16 current registers.
Port transition detection allows seven maskable ports
P24 through P30 to be continuously monitored for
changes in their logic status (Figure 6). Enable transi-
tion detection by setting the M bit in the configuration
register (Table 8) after setting the mask register. If port
31 is configured as an output (Tables 1 and 2), then
P31 automatically becomes an interrupt request (IRQ)
output to flag detected transitions. Port 31 can be con-
figured and used as a general-purpose input port
instead if not required as the IRQ output.
The mask register determines which of the seven ports
P24 through P30 are monitored (Table 13). Set the
appropriate mask bit to enable that port for transition
detect. Clear the mask bit if transitions on that port are
to be ignored by the transition detection logic. Ports are
monitored regardless of their I/O configuration, both
input and output.
The MAX6957 maintains an internal 7-bit snapshot reg-
ister to hold the comparison copy of the logic states of
ports P24 through P30. The snapshot register is updat-
ed with the condition of P24 through P31 whenever the
4-Wire-Interfaced, 2.5V to 5.5V, 20-Port and
28-Port LED Display Driver and I/O Expander
10
Transition (Port Data Change) Detection
in the last data bit), positions D7 through D0 in the
Shift register are now loaded with the register data
addressed by bits D1 through D8.
be a No-Op), and examine the bit stream at DOUT;
the second 8 bits are the contents of the register
addressed by bits D1 through D8 in step 3.
______________________________________________________________________________________
LED Current Control
Initial Power-Up
configuration register is written with the M bit set. The
update action occurs regardless of the previous state
of the M bit so that it is not necessary to clear the M bit
and then set it again to update the snapshot register.
When the data change detection bit is set, the
MAX6957 continuously compares the snapshot register
against the changing states of P24 through P31. When
a difference occurs, the IRQ bit (mask register bit D7) is
set and IRQ port P31 goes high if it is configured as an
output.
The IRQ bit and IRQ output remain set until the mask
register is next read or written, so if the IRQ is set, then
the mask register reads with bit D7 set. Writing the
mask register clears the IRQ output and resets the IRQ
bit, regardless of the value of bit D7 written.
Display test mode turns on all ports configured as LED
drivers by overriding, but not altering, all controls and
port registers, except the port configuration register
(Table 14). Only ports configured as LED drivers are
affected. Ports configured as GPIO push-pull outputs
do not change state. In display test mode, each port's
current is temporarily set to 1/2 the maximum current
limit as controlled by R
The MAX6957 uses an external resistor R
maximum segment current. The recommended value,
39kΩ, sets the maximum current to 24mA, which makes
the segment current adjustable from 1.5mA to 24mA in
1.5mA steps.
To set a different segment current, use the formula:
where I
The recommended value of R
The recommended value of R
allowed value, since it sets the display driver to the
maximum allowed segment current. R
higher value to set the segment current to a lower maxi-
mum value where desired. The user must also ensure
that the maximum current specifications of the LEDs
connected to the driver are not exceeded.
The drive current for each segment can be controlled
through programming either the global current register
(Table 9) or individual segment current registers
(Tables 10, 11, and 12), according to the setting of the
current control bit of the configuration register (Table 7).
SEG
Selecting External Component R
is the desired maximum segment current in mA.
to Set Maximum Segment Current
R
ISET
Display Test Register
ISET
= 936kΩ / I
.
ISET
ISET
is 39kΩ.
SEG
is the minimum
ISET
ISET
can be a
to set the
ISET