DS232B Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 13 of 25

FT232BM USB UART ( USB - Serial) I.C.

7.0 Device Configuration Examples

7.1 Oscillator Configurations

Figure 4

3-Pin Ceramic Resonator

Configuration

Figure 5

Crystal or 2-Pin Ceramic Resonator

Configuration

XTIN

XTOUT

27

28

FT223BM

3-Pin Resonator

6MHz

1M

XTIN

XTOUT

27

28

FT232BM

2 - Pin Resonator

or Crystal

6MHz

27pF

27pF

Figure 4 illustrates how to use the FT232BM 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 MegaOhm 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 out-with the USB

specification, but it has been calculated that using such a device will work satisfactorily in practice with a FT232BM

design.

Figure 5 illustrates how to use the FT232BM 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.