Low Power V.32 bis Modem
CMX869
5.1
Tx USART
A flexible Tx USART is provided for all modem modes, that can be programmed to transmit continuous
patterns, Start-Stop characters and Synchronous unformatted data.
In Synchronous Data and Start-Stop modes the data to be transmitted is written by the µC into the C-BUS
Tx Data Register. The Tx (and Rx) Data Register can be set to operate in 8 or 16-bit mode by setting the
General Control Register b10 appropriately.
In 16-bit (2 character) mode data written to the Tx Data Register at C-BUS address $E3 will be treated as
two octets, b15-8 which will be transmitted first and b7-0 which will be transmitted second. If there is a
need to transmit a single octet when the Tx Data Register has been set to 16-bit mode this can be
achieved by writing the 8-bit data to C-BUS address $E4 instead of $E3.
If Synchronous Data mode has been selected the 8 data bits of each octet in the Tx Data Buffer are
transmitted serially, the lsb being sent first.
In Start-Stop mode an asynchronous character is transmitted for each octet in the Tx Data Register. Each
character consists of a single Start bit followed by 5, 6, 7 or 8 data bits from the Tx Data Buffer - lsb first -
followed by an optional Parity bit then - normally - one or two Stop bits. The Start, Parity and Stop bits are
generated by the USART as determined by the Tx Mode Register settings and are not taken from the Tx
Data Register.
Figure 5a Tx USART (in 16 Bit Mode)
Figure 5b Tx USART Output: Start-Stop mode, 8 Data Bits + Parity
© 2004 CML Microsystems Plc
12
D/869/4
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