Low-Power V.32 bis Modem
CMX869B
4.
External Components
DV
DD
R1
IRQN
DV
DD
REPLY DATA
RDRVN
DV
SS
C-BUS SERIAL CLOCK
to/from COMMAND DATA
µC
CSN
AV
SS
RXA
Rx Line
Interface
RXBN
RXAN
RXAFB
AV
DD
+
C1
AV
SS
C2
24
23
22
3
21
4
20
5
6
CMX869B
19
18
7
17
8
16
9
10
15
11
14
12
13
1
2
C-BUS to µC
C5
X1
+
C8
C6
DV
SS
XTALN
XTAL/CLOCK
V
DEC
C7
DV
SS
NC
RDN
AV
SS
TXA
TXAN
V
BIAS
C9
AV
SS
Tx Line
Interface
DV
DD
+
C3
DV
SS
C4
R1
X1
C1, C3, C8
100kΩ
6.144MHz
10uF
C2, C4, C7, C9
C5, C6
100nF
47pF (see text)
Resistors
±5%,
capacitors
±20%
unless otherwise stated.
Figure 2a Recommended External Components for a Typical Application
This device is capable of detecting and decoding small amplitude signals. To achieve this DV
DD,
AV
DD
and V
BIAS
should be decoupled and the receive path protected from extraneous in-band signals. It is
recommended that the printed circuit board is laid out with both AV
SS
and DV
SS
ground planes in the
CMX869B area, as shown in Figure 2b, with provision to make a link between them close to the
CMX869B. To provide a low impedance connection to ground, the decoupling capacitors (C1 – C4, C7,
C8) must be mounted as close to the CMX869B as possible and connected directly to their respective
ground plane. This will be achieved more easily by using surface mounted capacitors.
V
BIAS
is used as an internal reference for detecting and generating the various analogue signals. It must
be carefully decoupled, to ensure its integrity. If V
BIAS
needs to be used (other than as shown in figures
4a or 4b) to set external analogue levels, it must be buffered with a high input impedance buffer.
The values for capacitors C5 and C6 are suggestions for use with many typical crystals. However, the
values of these capacitors must be chosen to comply with the crystal manufacturer’s specification to
ensure that the clock accuracy is within 50ppm. The DV
SS
connections to the Xtal oscillator capacitors C5
and C6 should also be of low impedance and preferably be part of the DV
SS
ground plane to ensure
reliable start up of the oscillator.
©
2008 CML Microsystems Plc
7
D/869B/3
CMLMICRO [ CML MICROCIRCUITS ]
