Low Power V.22 bis Modem
CMX868
In order that received Break signals can be handled correctly in V.14 Rx overspeed mode, a received
character which has all bits ‘0’, including the Stop and any Parity bits, will always cause the Rx Framing
Error bit to be set and the USART to re-synchronise onto the next ‘1’ – ‘0’ transition. Additionally the
received Continuous 0s detector will respond when more than 2M + 3 consecutive ‘0’s are received,
where ‘M’ is the selected total number of bits per character including Stop and any Parity bits.
1.5.10 C-BUS Interface
This block provides for the transfer of data and control or status information between the CMX868’s
internal registers and the µC over the C-BUS serial bus. Each transaction consists of a single Register
Address byte sent from the µC which may be followed by a one or more data byte(s) sent from the µC to
be written into one of the CMX868’s Write Only Registers, or a one or more byte(s) of data read out from
one of the CMX868’s Read Only Registers, as illustrated in Figure 9.
Data sent from the µC on the Command Data line is clocked into the CMX868 on the rising edge of the
Serial Clock input. Reply Data sent from the CMX868 to the µC is valid when the Serial Clock is high.
The CSN line must be held low during a data transfer and kept high between transfers. The C-BUS
interface is compatible with most common µC serial interfaces and may also be easily implemented with
general purpose µC I/O pins controlled by a simple software routine. Figure 15 gives detailed C-BUS
timing requirements.
The following C-BUS addresses and registers are used by the CMX868:
General Reset Command (address only, no data).
General Control Register, 16-bit write only.
Transmit Mode Register, 16-bit write-only.
Receive Mode Register, 16-bit write-only.
Transmit Data Register, 8-bit write only.
Receive Data Register, 8-bit read-only.
Status Register, 16-bit read-only.
Address $01
Address $E0
Address $E1
Address $E2
Addresses $E3 and $E4
Address $E5
Address $E6
Programming Register, 16-bit write-only.
Address $E8
Note: The C-BUS addresses $E9, $EA and $EB are allocated for production testing and should not be
accessed in normal operation.
1.5.10.1 General Reset Command
General Reset Command
(no data)
C-BUS address $01
This command resets the device and clears all bits of the General Control , Transmit Mode and Receive
Mode Registers and bits 15 and 13-0 of the Status Register.
Whenever power is applied to the CMX868 a General Reset command should be sent to the device, after
which the General Control Register should be set as required.
ã 2004 CML Microsystems Plc
18
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
