8
Display Overview
The HCMS-39XX series is a family of LED displays driven
by on-board CMOS ICs. The LEDs are congured as 5x7
font characters and are driven in groups of 4 characters
per IC. Each IC consists of a 160-bit shift register (the Dot
Register), two 7-bit Control Words, and refresh circuitry.
The Dot Register contents are mapped on a one-to-one
basis to the display. Thus, an individual Dot Register bit
uniquely controls a single LED.
Eight-character displays have two ICs that are cascaded.
The Data Out line of the rst IC is internally connected to
the Data In line of the second IC forming a 320-bit Dot
Register. The display’s other control and power lines are
connected directly to both ICs.
Reset
Reset initializes the Control Registers (sets all Control
Register bits to logic low) and places the display in the
sleep mode. The Reset pin should be connected to the
system power on reset circuit. The Dot Registers are not
cleared upon power-on or by Reset. After power-on,
the Dot Register contents are random; however, Reset
will put the display in sleep mode, thereby blanking the
LEDs. The Control Register and the Control Words are
cleared to all zeros by Reset.
To operate the display after being Reset, load the Dot
Register with logic lows. Then load Control Word 0 with
the desired brightness level and set the sleep mode bit
to logic high.
Dot Register
The Dot Register holds the pattern to be displayed by the
LEDs. Data is loaded into the Dot Register according to
the procedure shown in Table 1 and Figure 5.
First RS is brought low, then CE is brought low. Next, each
successive rising CLK edge will shift in the data at the D
IN
pin. Loading a logic high will turn the corresponding
LED on; a logic low turns the LED o. When all 160 bits
have been loaded (or 320 bits in an 8-digit display), CE is
brought to logic high.
When CLK is next brought to logic low, new data is
latched into the display dot drivers. Loading data into
the Dot Register takes place while the previous data is
displayed and eliminates the need to blank the display
while loading data.
Table 1. Register Truth Table
Function CLK CE RS
Select Dot Register Not Rising p L
Load Dot Register n L X
D
IN
= HIGH LED = “ON”
D
IN
= LOW LED = “OFF”
Copy Data from Dot Register to Dot Latch L H X
Select Control Register Not Rising p H
Load Control Register
[1,3]
n L X
Latch Data to Control Word
[2]
L H X
Notes:
1. BIT D
0
of Control Word 1 must have been previously set to Low for serial mode or High for simultaneous mode.
2. Selection of Control Word 1 or Control Word 0 is set by D
7
of the Control Shift Register. The unselected control word retains its previous value.
3. Control Word data is loaded Most Signicant Bit (D
7
) rst.
