V.22 bis Modem with AT Commands
CMX866
©
2008 CML Microsystems Plc
38
D/866/5
1.6 Application
Notes
1.6.1
Hardware Interface
The CMX866 can be interfaced to an external host µC, as shown in Figure 11, or to an RS232 computer
port (via a level shifter IC, to generate ±6V), as shown in Figure 12. These are complete phone line
interface circuits for operation at V
DD
= 5.0V. Figure 12 includes the additional gyrator components which
may be required for CTR21 compliance.
It is recommended that the full, 9-wire interface should be used, where practical. This is illustrated
functionally in Figure 10a. In some applications it will be necessary to move the safety barrier to the serial
port interface of the CMX866. If opto-couplers are used to achieve the safety isolation required, a
reduced RS232 interface, shown in Figure 10b, may be preferred for economic reasons.
Figure 10a Full RS232 Link
Figure 10b Reduced RS232 Link
The TXD signal can be used to wake up the CMX866 from Powersave or 'Zero-Power' states, as shown
above. The external host µC should ignore CTSN and write a character byte to the CMX866 via the TXD
pin, which is connected to the DTRN pin. This will wake up the CMX866 because the DTRN pin is pulled
low. The byte written, and any further activity on the TXD pin for up to 30ms when coming out of 'Zero-
Power' state (or up to 10µs when coming out of Powersave state), will be corrupted. Normal data transfer
can commence when the CTSN pin goes low, providing the RTSN pin is already low.
If the DTRN pin is not connected to the TXD pin, the TXD signal cannot be used on its own to wake up
the CMX866. If the DTRN pin is permanently wired low it will not be possible to wake up the CMX866
from an external host µC. Moreover, it will not be possible to put the CMX866 into either a Powersave or
'Zero-Power' state in the first place, so the device will remain permanently operational.