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MX867D2

Low Power V.22 Modem

MX-COM Inc  

Low Power V.22 Modem

10

CMX867 Advance Information

4.1 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.

C-BUS Interface

Tx Data Register

7

0

Tx data

from µC

Tx Data Buffer

Parity bit generator

Start/Stop

Tx USART

bits

USART Control

Modem bit rate clock

Scrambler

Enable

To FSK or

DPSK

Modulator

Continuous

patterns

Figure 6: 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.

Tx Line Signal:

Start B0 B1

B7 Par'y Stop

Tx Data Ready flag bit:

Figure 7: 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 Start-

Stop characters can be transmitted at up to 1% overspeed (basic signaling rate range) or 2.3% overspeed

(extended signaling rate range) by deleting a Stop bit from no more than one out of every 8 (basic range) or 4

(extended range) consecutive transmitted characters.

To accommodate the V.14 requirement the Tx Data Register has been given two C-BUS addresses, $E3 and

$E4. Data should normally be written to $E3.

In DPSK Start-Stop modes if data is written to $E4 then the programmed number of Stop bits will be reduced

by one for that character. In this way the µC can delete transmitted Stop bits as needed.

In FSK Start-Stop modes, data written to $E4 will be transmitted with a 12.5% reduction in the length of the

Stop bit at the end of that character.

2000 MX-COM, Inc.

www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054

Doc. # 20480219.001

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA

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