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參數(shù)資料
| 型號： | APPLICATIONHINT36 |
| 英文描述： | Infrared to RF Data Link |
| 中文描述： | 紅外射頻數(shù)據(jù)鏈 |
| 文件頁數(shù)： | 3/4頁 |
| 文件大?。?/td> | 39K |
| 代理商： | APPLICATIONHINT36 |
Application Note 2
Micrel
7-72
1997
clears the internal latches to turn all outputs off. Program-
ming is accomplished by addressing an output, presenting
the desired data (1 = ON, 0 = OFF), and strobing CHIP
SELECT with a logic low. DATA is transferred to the
addressed output on the falling edge of CHIP SELECT, and
is latched in place when CHIP SELECT returns to a high
state. In larger displays, CHIP SELECT serves as a means
of controlling several MIC4807s while the address, OUT-
PUT ENABLE, CLEAR, and DATA lines are paralleled.
For bench testing purposes a personal or laptop/portable
computer is quite useful. A parallel printer port is commonly
available and serves as a convenient means of program-
ming one or more MIC4807s. Software changes can be
made quickly and easily and, depending on the program-
ming language used, the program can be stepped manually
so that each bit can be checked “on the fly.” This presents
no problems because the MIC4807 is fully static.
An evaluation program written in BASIC is listed in Figure
4. The program consists of 5 parts. The control/input
section is lines 100 through 130. This portion scans the
keyboard, and branches to other parts of the program
depending on which key is pressed. A “l(fā)ine return” branches
to lines 3000 through 3030 where the MIC4807 is cleared
and the computer’s record of the MIC4807 latch states [8-
element array D(A)] is cleared. Execution then returns to
lines 100 through 130. A “” invokes a lamp test function—
all of the outputs are turned on by lines 2000 through 2060.
Pressing any other key reprograms the MIC4807 with the
original data, and returns execution to lines 100 through
130. Pressing any number from 1 to 8 toggles the associ-
ated output on or off (lines 1000 through 1020). Lines 4000
through 4020 are accessed from several points in the
program; these lines write data to a given address by
toggling CHIP SELECT.
The parallel output word is given a value according to which
MIC4807 pins should be high or low at any given time. A
in
has a numeric (decimal) value of 1, B
in
= 2, C
in
= 4,
DATA = 8, CHIP SELECT = 16, and CLEAR = 32 to repre-
sent a logical “1” at each pin. The port number (8) specified
in the “OUT” statements will vary from computer to com-
puter. While final evaluation of data communications must
be carried out with the actual host processor, using a
computer during the debugging phase of the display design
is most helpful.
An equivalent block diagram of the MIC4807 logic circuitry
is shown in Figure 5. Note that CHIP SELECT, DATA,
CLEAR, AND OUTPUT ENABLE operate on all channels in
parallel. The address decoder determines to which latch
CHIP SELECT is directed. DATA has no effect on the other
latches as their clocking signals remain low.
Display Dimming
Dimming is achieved by pulse-width modulation applied to
the OUTPUT ENABLE (OE) pin. Since OUTPUT ENABLE
acts on all 8 channels, the lamps are simultaneously dimmed
by one control signal and maintain equal brightness, re-
gardless of the dimming level.
An LM358 dual op-amp forms the basis of a variable PWM.
The control range extends from completely off to com-
pletely on, and to any intermediate brightness level.
The PWM frequency (400Hz) is considerably higher than
the filament’s thermal time constant, so the filament’s
resistance (and temperature) changes very little between
“on” and “off” periods. Figure 3 shows the pulsed filament
current in a PWM application for a single #1835 lamp as a
function of duty cycle. Lamp manufacturers recommend a
PWM frequency of at least 400Hz to eliminate aging effects
associated with thermal cycling. At an extremely dim 10%
duty cycle, a #1835 lamp accepts current pulses of 90mA on
a 55V supply, exhibiting a filament resistance of 611
. At
100% duty cycle the current falls to 50mA, at a resistance
of 1100
. In any dimming circuit the driver circuitry must be
sized to deliver the pulsed, low duty cycle current required
by the relatively cool filament. This is typically twice the
rated (100% duty cycle) lamp current.
MIC4807 Programming
The MIC4807 programming interface consists of a 3-bit
address, a data line, and two control lines (see Figure 2).
CLEAR is straightforward; a low on this pin asynchronously
0
20
40
60
80
100
DUTY CYCLE (%)
Figure 3. Pulsed Filament Current vs. Duty Cycle
100
80
60
40
0
20
P
#1835 LAMP
V = 55V
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