參數(shù)資料
型號(hào)： M494
廠商： 意法半導(dǎo)體
英文描述： SINGLE-CHIP VOLTAGE TUNING SYSTEM WITH 4 ANALOG CONTROLS AND mP INTERFACE
中文描述： 單片電壓調(diào)整系統(tǒng)，4模擬控制和MP接口
文件頁(yè)數(shù)： 5/22頁(yè)
文件大?。?/td> 223K
代理商： M494
τ
1
= RC
τ
2
= Ri . C
1.5V
4
Figure 3
FUNCTIONAL DESCRIPTION
(continued)
Several tests are performed on the signal :
a) Measurement of the pulse distance T (time
base synchronization).
b) Check of the bit positions relative to the time
basewindows.
c)
Check of the parity bit.
d) Checkof theabsenceof pulsesbetweenparity
and stop pulses.
e) Check of the noise level. The receiver checks
the noise level for a time T after each pulse
detected.
If all these tests are successfulthe receivedword
is stored and decoded. If not it is rejected. The
transmissionis terminatedon receptionof the end
of transmission (EOT) code or if the internal timer
measures atransmission interruptionof morethan
550ms. For more detail concerning the operation
of the RC receiver refer to SGS-THOMSON Tech-
nical Note No. 155 pp11-12.
The RC receiverand the local keyboard have the
same command source priority i.e. a local com-
mand is not accepted until a previously accepted
RC command has been completelyexecuted and
the EOTcode transmitted. Similarly if a localcom-
mand isunderexecutionthenan RCcommand will
not be accepted. The RC truth table and com-
mands are shown on the nextpage.
Analog Control Outputs
Four analog control outputs are implemented to
provide for Volume, Brightness, Saturation and
Contrast from four 6 bit D/A’s. TheseD/A’s use the
Pulse Width Modulationtechnique to synthesizea
pulse trainof constantfrequencybutvariablepulse
width (PWM). Each output delivers a 7.8kHz
square wave whose duty cycle is variable in 63
steps. External RC filtering and level shifting is
required to realise a static DC voltage from the
pulse train. If the analog outputs are continuously
varied by command from the keyboard or data
command sources theoutputswill change approx.
every 112ms (fck = 500kHz) or approx. every
102ms if the command is issued from the RC
command source. One analog control is specifi-
callydesignedasa volumecontrolasmutecircuitry
is built in.
On start up resetthe analogcontrol outputsexcept
volume are enabled after a period of approx.
1.1 seconds.In the Standbyand Off states all the
analog control outputs arepulled to logicLO.
The normalise command reads the contents of
each analog memory sequentially to its corre-
spondingcounterand D/Aoutput.
Tuning Output
The tuning voltage is generated from a 13 bit
counter.The programmemorystoresthe 12MSB’s
of the tuning word. The range of the AFC circuitry
is at least 3 bits so the LSB of the tuning counter
doesnot affect the resolutionof tuning.
The contents of the counterare convertedusing a
PWM and a Bit Rate Multiplier (BRM) technique.
13 bits gives 8192 steps which yields a resolution
of approximately 3.9mVwith a max. tuning voltage
of 32V. This corresponds to a resolution of about
75kHz in the UHF band. The 5 MSB’s of the tuning
word areconverted usingPWM and the remaining
8 bits using BRM. Thus as the tuning word in-
creasesfromall zeroesthenumberof pulsesinone
period increases to 256 with all the pulses being
the same length (t
o
= 2
μ
s). For values larger then
256 PWM conversiontakes placewhere the num-
ber of pulses in one period stays constant at 256
but the width changes.
Whenthe pulsewidthreaches15t
o
twosuccessive
pulses ”link” together and the number of pulses
decreases(see Figure 4).
The pulse trainis fed to an externallow pass filter
to realise a DC voltage. The temperaturedepend-
ence of this system is predominantly the switching
times of the output pulses and as there are only a
maximum of256 pulsesin one periodthetempera-
ture stability is very good.
InStandbyandOffstatesthetuningoutputispulled
to logic LO.
256
No OF PULSES
TUNING
VOLTAGE
1V
31V
4
Figure 4
M494
5/22
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