FT245BQ USB FIFO ( USB - Parallel ) I.C.
7.0 Device Configuration Examples
7.1 Oscillator Configurations
Figure 4
Figure 5
3 Pin Ceramic Resonator Configuration
Crystal or 2-Pin Ceramic Resonator
Configuration
FT245BQ
27pF
FT245BQ
27
27
XTIN
XTIN
3-Pin Resonator
6MHz
2 - Pin Resonator
or Crystal
1M
6MHz
27pF
28
28
XTOUT
XTOUT
Figure 4 illustrates how to use the FT245BQ with a 3-Pin Ceramic Resonator. A suitable part would be a ceramic
resonator from Murata’s CERALOCK range. (Murata Part Number CSTCR6M00G15), or equivalent. 3-Pin ceramic
resonators have the load capacitors built into the resonator so no external loading capacitors are required. This
makes for an economical configuration. The accuracy of this Murata ceramic resonator is +/- 0.1% and it is specifically
designed for USB full speed applications. A 1 MOhm loading resistor across XTIN and XTOUT is recommended in
order to guarantee this level of accuracy.
Other ceramic resonators with a lesser degree of accuracy ( typically +/- 0.5% ) are technically outwith the USB
specification, but it has been calculated that using such a device will work satisfactorily in practice with a FT245BQ
design.
Figure 5 illustrates how to use the FT245BQ with a 6MHz Crystal or 2-Pin Ceramic Resonator. In this case, these
devices do not have in-built loading capacitors so these have to be added between XTIN, XTOUT and GND as
shown. A value of 27pF is shown as the capacitor in the example – this will be good for many crystals and some
resonators but do select the value based on the manufacturers recommendations wherever possible. If using a crystal,
use a parallel cut type. If using a resonator, see the previous note on frequency accuracy.
DS245BQ Version 1.7
© Future Technology Devices Intl. Ltd. 2005
Page 15 of 24
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