DB929B datasheet(24/51 Pages) CMLMICRO

24 / 51 page

4-Level FSK Modem Data Pump

Page 24 of 50

MX929B PRELIMINARY INFORMATION

www.mxcom.com Tel: 800 638 5577 336 744 5050

Fax: 336 744 5054

Doc. # 20480171.003

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

All trademarks and service marks are held by their respective companies.

4.5.3

Control Register

This 8-bit write-only register controls the modem's symbol rate, the response times of the receive clock

extraction, signal level measurement circuits, and the Frame Sync pattern recognition tolerance to inexact

matches.

76543210

Control Register

FSTOL

LEVRES

PLLBW

CKDIV

4.5.3.1

Control Register B7, B6: CKDIV - Clock Division Ratio

These bits control a frequency divider driven from the clock signal present at the XTAL pin, therefore

determining the nominal symbol rate. Because each symbol represents two bits, bit rates are 2x the symbol

rates. The table below shows how symbol rates of 2400/4800/9600 symbols/sec (4800/9600/19200bps) may

be obtained from common Xtal frequencies:

Xtal Frequency (MHz)

2.4576

4.9152

9.8304

Division Ratio:

Xtal Frequency/Symbol Rate

Symbol Rate (symbols/sec) / Bit Rate (bps)

512

4800/9600

9600/19200

1024

2400/4800

4800/9600

9600/19200

2048

2400/4800

4800/9600

4096

2400/4800

Note: Device operation is not guaranteed below 2400 symbols/sec (4800bps) or above 9600

symbols/sec (19200bps).

4.5.3.2

Control Register B5, B4: FSTOL - Frame Sync Tolerance to Inexact Matches

These two bits have no effect in transmit mode. In receive mode, they define the maximum number of

mismatches allowed during a search for the Frame Sync pattern:

Mismatches allowed

Note: A single 'mismatch' is defined as the difference between two adjacent symbol levels, thus if the symbol

'+1' were expected, then received symbol values of '+3' and '-1' would count as 1 mismatch, a received

symbol value of '-3' would count as 2 mismatches. A setting of '4 mismatches' is recommended for

normal use.

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