Low Power V.22 Modem
CMX867A
5.1
Tx USART
A flexible Tx USART is provided for all modem modes, meeting the requirements of V.14 for DPSK
modems.
It can be programmed to transmit continuous patterns, Start-Stop characters or Synchronous Data.
In both Synchronous Data and Start-Stop modes the data to be transmitted is written by the µC into the 8-
bit C-BUS Tx Data Register from which it is transferred to the Tx Data Buffer.
If Synchronous Data mode has been selected the 8 data bits in the Tx Data Buffer are transmitted serially,
b0 being sent first.
In Start-Stop mode a single Start bit is transmitted, followed by 5, 6, 7 or 8 data bits from the Tx Data
Buffer - b0 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
Every time the contents of the C-BUS Tx Data Register are transferred to the Tx Data Buffer the Tx Data
Ready flag bit of the Status Register is set to 1 to indicate that a new value should be loaded into the C-
BUS Tx Data Register. This flag bit is cleared to 0 when a new value is loaded into the Tx Data Register.
Figure 5b Tx USART Function (Start-Stop mode, 8 Data Bits + Parity)
If a new value is not loaded into the Tx Data Register in time for the next Tx Data Register to Tx Data
Buffer transfer then the Status Register Tx Data Underflow bit will be set to 1. In this event the contents of
the Tx Data Buffer will be re-transmitted if Synchronous Data mode has been selected, or if the Tx
modem is in Start-Stop mode then a continuous Stop signal (1) will be transmitted until a new value is
loaded into the Tx Data Register.
In all modes the transmitted bit and baud rates are the nominal rates for the selected modem type, with an
accuracy determined by the XTAL frequency accuracy, however for DPSK modes V.14 requires that
© 2008 CML Microsystems Plc
11
D/867A/3
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