ACS401 データシートの表示（PDF） - Semtech Corporation
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ACS401 Datasheet PDF : 8 Pages
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Control of LASER current
The LASER output current must be set for each individual device
in accordance with the manufacturer’s recommendations. The
maximum output current to the Laser is controlled by the resistor
Rtrc connected between ground and TRC. The minimum value set
on Rtrc to avoid damage to the ACS401 is 800Ω.
LASER(current max) = 100/Rtrc
tolerance +/- 25 %.
Whilst TRC sets the maximum current ( to prevent damage to the
laser during the adjustment procedure), the actual current to the
Laser is determined by the external component VR2. The
adjustment procedure is described in the following section.
Adjustment Procedure
Select the appropriate mode using the pins DP(1:4) (see section
headed Operational Modes). Choose a value for the resistor Rtrc
that delivers sufficient current to correctly drive the laser at the
desired power, with sufficient power margin to compensate for
temperature/voltage changes and potential laser degradation. In
many applications the value of Rtrc will be set to the maximum
current using a resistor value of 800 Ω.
The output power from the laser may be measured directly using
an optical-power meter that is capable of detecting peak optical-
power. If an average optical-power meter is employed then a
correction factor of 16 must be used to obtain the peak value .
LASER(peak power) = Laser(average power) * 16.
The external component VR2 should have a range of 0 to 50 KΩ.
As VR2 is decreased, output power will first be detected when the
laser begins to lase. VR2 should be reduced further until the
desired output power is achieved (within the limits of the
manufacturer’s specification).
LED current control
The LED transmit current is less critical though it is important not
to exceed the LED manufacturer’s recommendation for maximum
current. The current is controlled by a resistor Rtrc connected
between TRC and ground. The lower the value Rtrc, the greater
the current. The lower limit for Rtrc is 800 Ω while a practical
maximum is 40 kΩ.
The LED current is inversely proportional to Rtrc while Rtrc > 800Ω:
LED(current) = 100/Rtrc
tolerance +/- 25 %.
PORB
The Power-On Reset or PORB pin resets the device if forced Low
for 100 ms or more. In normal operation PORB should be held
High. Although the PORB pin has been included, e.g. for factory
test, the modem has been designed to power up correctly without
the aid of PORB.
PORB has a special function when used in conjunction with
memory lock (see section headed Diagnostic Modes).
Crystal Clock
Normally, a parallel resonant crystal will be connected between
the pins XLI and XLO with the appropriate padding capacitors.
Alternatively, it is possible to drive XLI directly by an external
clock.
The clock frequency for the purpose of this specification will be
known as XTAL frequency. The operational range for XTAL
frequency is 1 - 12 MHz, though communicating ACS401s must
be clocked at the same nominal frequency. The ACS401 has
been designed to operate with a XTAL tolerance of 100 ppm
giving a relative tolerance of 200 ppm between communicating
modems.
The recommended frequency of 9.216 MHz, results in the
standard range of synchronous communication frequencies
tabulated in the section headed Data Rate Selection. N o n -
standard frequencies may be generated by using the appropriate
value XTALor external clock.
Example: To generate a 38.4 kbps dual channel.
Select a 64 kbps dual channel using the data rate selection pins
DR4/3/2/1 = 1/1/0/0, and use a XTAL frequency of:
XTAL = (38.4 / 64) * 9.216 MHz = 5.5296 MHz.
Other ‘non-standard’ transmission frequencies may be generated
in the same way as long as the 1 - 12 MHz XTAL oscillator range
is observed. A wider range of external clock frequencies may also
be permissible - please check with Acapella.
Cint Capacitor - CNT
For a XTAL frequency range of 5 - 12 MHz the ACS401 requires
a ceramic capacitor of value 22 nF - 33 nF +/- 20 % between pin
CNT and GND. At frequencies lower than 5 MHz a capacitor of
value 68 nF - 100 nF is recommended. It is essential that the CNT
capacitor is placed very close to the ACS401.
DCDB
The Data Carrier Detect (DCDB) signal will go Low when the
modems are locked and ready for data transmission. Prior to lock
(DCDB = High), the data channels outputs RxD(1:4) are forced
Low with the control lines XO(1:2) forced High.
Data Rate Selection
The following data rates apply to TxD1, TxD2, TxD3 and TxD4 and
are based on the use of a 9.216 MHz XTALor clock.
‘Standard’ mode, DR4 = 1:
DR4
1
1
1
1
1
1
1
1
DR3
0
0
0
0
1
1
1
1
DR2
0
0
1
1
0
0
1
1
DR1
0
1
0
1
0
1
0
1
Data Rate
128 kbps
64 kbps
32 kbps
16 kbps
64 kbps
32 kbps
32 kbps
16 kbps
No. of
Channels
Single
Single
Single
Single
Dual
Dual
Four
Four
‘Double’mode, DR4 = 0:
DR4
0
0
0
0
0
0
DR3
0
0
0
1
0
1
DR2
0
1
1
0
0
1
DR1
1
0
1
1
0
1
Data Rate
64 kbps
32 kbps
16 kbps
32 kbps
16 kbps
16 kbps
No. of
Channels
Single
Single
Single
Dual
Dual
Four
In ‘standard’ mode internal timing accommodates fiber length
delays of up to 95 km, link budget permitting. In ‘double’mode the
internal timing accommodates fiber length delays of up to 190 km,
link budget permitting. ‘Double’ mode essentially halves the
frequency of the internal clock and therefore doubles the period of
the internal machine cycle, this also has the effect of doubling the
duration of LED/LASER transmit pulses, which in turn is likely to
lead to improved link budgets, particularly where LEDs or LASERs
of higher than recommended capacitance are employed.
Bandwidth * Channel Product
The ACS401 has a Bandwidth Channel Product (BCP) of 128 kHz
in ‘standard’ mode and 64 kHz in ‘double’ mode using the
recommended XTAL frequency of 9.216 MHz. The BCP is
proportional to the XTAL frequency and is specified as:
BCP(kHz) = 128 * XTAL / ( 9.216 *106 * n)
XTAL = frequency of XTALoscillator. n = 1 for ‘standard’mode; n
= 2 for ‘double’ mode.
The given maximum bandwidth may be shared by up to 4
channels as shown in the following tables for both ‘standard’ and
‘double’ modes and a XTAL frequency of 9.216 MHz.
‘Standard’ mode - XTAL = 9.216 MHz
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