Datasheet

Display Test Register
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 tempo-
rarily set to 1/2 the maximum current limit as controlled
by R
ISET
.
Selecting External Component R
ISET
to Set Maximum Segment Current
The MAX6957 uses an external resistor R
ISET
to set the
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:
RISET = 936kΩ/I
SEG
where I
SEG
is the desired maximum segment current in mA.
The recommended value of R
ISET
is 39kΩ.
The recommended value of R
ISET
is the minimum
allowed value, since it sets the display driver to the
maximum allowed segment current. R
ISET
can be a
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 regis-
ter (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).
These registers select the LED’s constant-current drive
from 16 equal fractions of the maximum segment current.
The current difference between successive current steps,
I
STEP
, is therefore determined by the formula:
I
STEP
= I
SEG
/16
If I
SEG
= 24mA, then I
STEP
= 24mA/16 = 1.5mA.
Note: Unused bits read as 0.
Table 3. Register Address Map (continued)
REGISTER
COMMAND ADDRESS
HEX
CODE
D15 D14 D13 D12 D11 D10 D9 D8
8 ports 16–23 (data bits D0–D7) R/W 1 0 1 0 0 0 0 0x50
8 ports 17–24 (data bits D0–D7) R/W 1 0 1 0 0 0 1 0x51
8 ports 18–25 (data bits D0–D7) R/W 1 0 1 0 0 1 0 0x52
8 ports 19–26 (data bits D0–D7) R/W 1 0 1 0 0 1 1 0x53
8 ports 20–27 (data bits D0–D7) R/W 1 0 1 0 1 0 0 0x54
8 ports 21–28 (data bits D0–D7) R/W 1 0 1 0 1 0 1 0x55
8 ports 22–29 (data bits D0–D7) R/W 1 0 1 0 1 1 0 0x56
8 ports 23–30 (data bits D0–D7) R/W 1 0 1 0 1 1 1 0x57
8 ports 24–31 (data bits D0–D7) R/W 1 0 1 1 0 0 0 0x58
7 ports 25–31 (data bits D0–D6. D7 reads as 0) R/W 1 0 1 1 0 0 1 0x59
6 ports 26–31 (data bits D0–D5. D6, D7 read as 0) R/W 1 0 1 1 0 1 0 0x5A
5 ports 27–31 (data bits D0–D4. D5–D7 read as 0) R/W 1 0 1 1 0 1 1 0x5B
4 ports 28–31 (data bits D0–D3. D4–D7 read as 0) R/W 1 0 1 1 1 0 0 0x5C
3 ports 29–31 (data bits D0–D2. D3–D7 read as 0) R/W 1 0 1 1 1 0 1 0x5D
2 ports 30–31 (data bits D0–D1. D2–D7 read as 0) R/W 1 0 1 1 1 1 0 0x5E
1 port 31 only (data bit D0. D1–D7 read as 0) R/W 1 0 1 1 1 1 1 0x5F
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MAX6957 4-Wire-Interfaced, 2.5V to 5.5V, 20-Port and
28-Port LED Display Driver and I/O Expander