Echelon LONWORKS PLT-22, Руководство пользователя

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External Components
Crystal
The PLT-22 transceiver requires the connection of an external 10.0000MHz crystal.
The crystal connects directly to two pins of the PLT-22 transceiver, with no other
oscillator components being required external to the transceiver. The crystal should
be mounted as close as possible to the transceiver to minimize parasitic effects. The
traces connecting the crystal to the transceiver preferably should be less than 10mm
(0.4") in length, and under no circumstances can they exceed 20mm (0.8") in length.
In addition to the nominal frequency specification of 10.0000MHz, the crystal should
be a parallel resonant type with a load rating of 13 to 20pF (series resonant crystals
should not be used). The frequency accuracy of the oscillator must be held to ±200
ppm over the full temperature range of operation. The PLT-22 oscillator design is
centered for the use of a crystal with a 16pF load rating. If a 13pF crystal is used,
the nominal frequency of oscillation will be 30 to 55ppm low, leaving 145ppm
available for crystal accuracy, temperature, and aging. If a 20pF crystal is used, the
nominal frequency will be 50 to 85ppm high, leaving 115 ppm for accuracy,
temperature, and aging.
While the PLT-22 transceiver design is centered around the use of a crystal whose
load capacitance rating is 16pF, it is possible to recenter the design for a crystal
whose load capacitance rating is higher than 16pF. The PLT-22 transceiver
effectively contains two 32pF capacitors, one from XIN to ground and one from
XOUT to ground. Note that the series combination of the two capacitors equals the
load capacitance of the recommended crystal. Recentering the design for the use of a
crystal with a higher load capacitance specification requires the addition of two
external capacitors whose series equivalent capacitance is equal to the load
capacitance specification of the crystal less 16pF. For instance, to use a crystal with
a load capacitance specification of 30pF, the operating frequency can be centered by
adding two 27pF capacitors, one from XIN to ground and one from XOUT to ground.
If a 10.0000MHz ±200ppm 0-5V clock signal is already available as part of the node
hardware then it may be used as a clock source for the PLT-22 transceiver by
connecting it to Pin 18 (XIN) of the transceiver. In this instance pin 19 (XOUT) of
the PLT-22 should be left unconnected. With this option, appropriate high speed
clock distribution techniques must be strictly followed in order to ensure that a clean
clock signal is present at the XIN pin of the PLT-22 transceiver.
Oscillator frequency accuracy should be checked during the design verification phase
of every PLT-22 based product. Frequency accuracy should be measured by using a
frequency counter connected to the CKOUT pin to compare the frequency of that pin
to the selected value (i.e., 10, 5, 2.5, or 1.25MHz). Be sure that no instruments or
extra cabling are connected to either the XIN or XOUT pins of the PLT-22 since even
2pF of extra load on either pin will significantly change the oscillator frequency.
2-6 Using the PLT-22 Transceiver