HCMS-2913 datasheet(15/16 Pages) HP

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be varied between 4.0 to 5.5 V

without any noticeable variation

in light output. However, operat-

ing V

LED below 4.0 V may cause

objectionable mismatch between

the pixels and is not

recommended. Dimming the

display by pulse width modulating

LED is also not recommended.

LOGIC can vary from 3.0 to 5.5 V

without affecting either the

displayed message or the display

intensity. However, operation

below 4.5 V will change the

timing and logic levels and

operation below 3 V may cause

the Dot and Control Registers to

be altered.

The logic ground is internally

connected to the LED ground by a

substrate diode. This diode

becomes forward biased and

conducts when the logic ground is

0.4 V greater then the LED

ground. The LED ground and the

logic ground should be connected

to a common ground which can

withstand the current introduced

by the switching LED drivers.

When separate ground

connections are used, the LED

ground can vary from -0.3 V to

+0.3 V with respect to the logic

ground. Voltages below -0.3 V can

cause all the dots to be ON.

Voltage above +0.3 V can cause

dimming and dot mismatch. The

LED ground for the LED drivers

can be routed separately from the

logic ground until an appropriate

ground plane is available. On long

interconnections between the

display and the host system,

voltage drops on the analog

ground can be kept from affecting

the display logic levels by

isolating the two grounds.

Electrostatic Discharge

The inputs to the ICs are pro-

tected against static discharge

and input current latchup. How-

ever, for best results, standard

CMOS handling precautions

should be used. Before use, the

HCMS-29XX should be stored in

antistatic tubes or in conductive

material. During assembly, a

grounded conductive work area

should be used and assembly

personnel should wear conductive

wrist straps. Lab coats made of

synthetic material should be

avoided since they are prone to

static buildup. Input current

latchup is caused when the CMOS

inputs are subjected to either a

voltage below ground (V

IN <

ground) or to a voltage higher

then V

LOGIC (VIN > VLOGIC) and

when a high current is forced into

the input. To prevent input

current latchup and ESD damage,

unused inputs should be con-

nected to either ground or V

LOGIC.

Voltages should not be applied to

the inputs until V

LOGIC has been

applied to the display.

Appendix C. Oscillator

The oscillator provides the

internal refresh circuitry with a

signal that is used to synchronize

the columns and rows. This

ensures that the right data is in

the dot drivers for that row. This

signal can be supplied from either

an external source or the internal

source.

A display refresh rate of 100 Hz

or faster ensures flicker-free

operation. Thus for an external

oscillator the frequency should be

greater than or equal to 512 x

100 Hz = 51.2 kHz. Operation

above 1 MHz without the

prescaler or 8 MHz with the

prescaler may cause noticeable

pixel to pixel mismatch.

Appendix D. Refresh

Circuitry

This display driver consists of 20

one-of-eight column decoders and

20 constant current sources, 1

one-of-eight row decoder and

eight row sinks, a pulse width

modulation control block, a peak

current control block, and the

circuit to refresh the LEDs. The

refresh counters and oscillator are

used to synchronize the columns

and rows.

The 160 bits are organized as 20

columns by 8 rows. The IC

illuminates the display by

sequentially turning ON each of

the 8 row-drivers. To refresh the

display once takes 512 oscillator

cycles. Because there are eight

row drivers, each row driver is

selected for 64 (512/8) oscillator

cycles. Four cycles are used to

briefly blank the display before

the following row is switched on.

Thus, each row is ON for 60

oscillator cycles out of a possible

64. This corresponds to the

maximum LED on time.

Appendix E. Display

Brightness

Two ways have been shown to

control the brightness of this LED

display: setting the peak current

and setting the duty factor. Both

values are set in Control Word 0.

To compute the resulting display

brightness when both PWM and

peak current control are used,

simply multiply the two relative

brightness factors. For example,

if Control Register 0 holds the

word 1001101, the peak current

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