V.22 bis Modem with AT Commands
CMX866
1.5.5 Tx USART
A flexible Tx USART is provided for all CMX866 modes. 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 on-chip
µController into the 8-bit DSP 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
in the order in which they are received, ie. 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 DSP Tx Data Register.
Figure 6a Tx USART
Every time the contents of the DSP Tx Data Register are transferred to the Tx Data Buffer, the Tx Data
Ready flag bit of the DSP Status Register is set to 1 to indicate that a new value should be loaded into
the DSP Tx Data Register. This flag bit is cleared to 0 when a new value is loaded into the DSP Tx Data
Register. The interpretation of this flag is handled automatically by the on-chip µController.
Figure 6b Tx USART Function (Start-Stop mode, 8 Data Bits + Parity)
If a new value is not loaded into the DSP Tx Data Register in time for the next DSP Tx Data Register to
Tx Data Buffer transfer then the DSP 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 Start-Stop mode was selected then a continuous Stop signal (1) will be transmitted until a new value is
loaded into the DSP 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. Underspeed transmission is automatically
handled by the CMX866 in Start-Stop mode as it inserts extra Stop bit(s) if it has to wait for new data to
be loaded into the DSP Tx Data Register.
ã 2004 CML Microsystems Plc
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D/866/4
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