Data Sheet AS1100
Revision 1.32, Oct. 2004 Page 8 of 12
Display T est Regis ter
With the display test register 0Fh all LED can be tested. In
the test mode all LEDs are switched on at maximum
brightness (duty cycle 31/32). All programming of digit and
control registers is maintained. The format of the register is
given in table 10.
Number of
Digits
Displayed
Maximum
Segment Current
(mA)
110
220
330
Tabl e 9: Maxim um segme nt current for 1-, 2-, or 3-d igit displa ys
Register Data
Mode D7 D6 D5 D4 D3 D2 D1 D0
Normal OperationXXXXXXX 0
Display Test
Mode XXXXXXX1
Table 10: Display-test register format (address (hex) = 0xXF)
Note : The AS1100 remains in display-test mode until the
display-test register is reconfigured for normal operation.
No-Op Register (Ca scading o f As1100)
The no-operation register 00h is used when AS1100s are
cascaded in order to support more than 8 digit displays. The
cascading must be done in a way that all DOUT are
connected to DIN of the following AS1100. The LOAD and
CLK signals are connected to all devices. For a write
operation for example to the fifth device the command must
be followed by four no-operation commands. When the
LOAD signal finally goes to high all shift registers are
latched. The first four devices have got no-operation
commands and only the fifth device sees the intended
command and updates i ts registe r.
Reset an d external Clock Regi ster3
This re gister is a ddressed vi a the se rial interfac e. It allo ws
to switc h the device to extern al clock mod e (If D0= 1 the
CLK pin of the serial int erface operates as syste m clock
input.) and to apply an external reset (D 1). This bri ngs all
regist ers (excep t reg. E) to default sta te. For sta ndard
operati on the regis ter contents should b e "00h".
3 This register is not used by MAX7219, since it does not support
software reset and external clocks
Register Data
Mode Address
code (hex) D7 D6 D5 D4 D3 D2 D1 D0
Normal O peration,
internal c lock 0xXE XXXXXX00
Normal O peration,
external c lock 0xXE XXXXXX01
Reset state,
internal c lock 0xXE XXXXXX10
Reset state,
external c lock 0xXE XXXXXX11
Table 11: Reset and external Clock register (address (hex) = 0xXE)
Applications Information
Supply Bypassing and Wiring
In order to achieve optimal performance the AS1100 shall
be placed very close to the LED display to minimize effects
of electromagnetic interference and wiring inductance.
Furthermore it is recommended to connect a 10µF
electrolytic and a 0.1µF ceramic capacitor between VDD
and GND to avoid power supply ripple. Also, both GNDs
must be c onnected t o ground.
Selecting RSET Resistor and Using External Drivers
The current through the segments is controlled via the
external resistor RSET. Segment current is about 100 times
the current in ISET. The right values for ISET are given in
table 12. The maximum current the AS1100 can drive is
40mA. If higher currents are needed, external drivers must
be used. In that case it is no longer necessary that the
AS1100 drives high currents. A recommended value for
RSET is 47kΩ. In cases that the AS1100 only drives few
digits table 9 specifies t he maximum currents and RSET must
be set accordingly. Refer to absolute maximum ratings to
calculate acceptable limits for ambient temperature,
segment c urrent, an d the LED for ward-volta ge drop.
VLED (V)
ISEG (m A) 1.5 2.0 2.5 3.0 3.5
40 12.2 11.8 11.0 10.6 9.69
30 17.8 17.1 15.8 15.0 14.0
20 29.8 28.0 25.9 24.5 22.6
10 66.7 63.7 59.3 55.4 51.2
Tabl e 12: RSET vs. seg ment current and LE D for ward volt age