V23 / Bell 202 Modem

CMX624

 2003 CML Microsystems Plc

8

D/624/7

1.5.4

FSK Demodulator

This block is enabled when bits 1 and 5 of the FSK MODE Register are set to ‘1’, and converts the 75, 150

or 1200 bps FSK input signal to a binary received data signal which is sent to the Rx UART block.

Note that in the absence of a valid FSK signal, the demodulator may falsely interpret speech or other

extraneous signals as data.

1.5.5

Rx Energy and 2100Hz Detector

The function of this block is controlled by Bits 4 and 5 of the FSK MODE Register and Bit 0 of the TX

TONES Register.

When Bit 0 of the TX TONES Register and Bits 4 and 5 of the FSK MODE Register are set to ‘1’ this block

will measure the frequency and amplitude of the incoming signal. When a signal of 2100Hz is present of

sufficient amplitude and time Bit 4 of the FLAGS Register is set high. See Section 1.7.1 for amplitude,

time and frequency limits.

When Bit 0 of the TX TONES Register is set to ‘0’ this block compares the level of the signal at the output

of the Receive Filter against an internal threshold and may be used as a FSK level detector or a simple

Call Progress Signal detector according to the settings of bits 4 and 5 of the FSK MODE Register, which

affect the Receive Filter pass band as described in Section 1.5.3.

The required register settings are summarised in the table below:

TX TONES Reg

FSK MODE Reg

Bit 0

Bit 5

Bit 4

Detection Mode

0

0

0

Call Progress

0

1

0

75 / 150 bps FSK

0

1

1

1200 bps FSK

1

1

1

2100 Hz

Bit 4 of the FLAGS Register is set to ‘1’ by the output of this block when the received level has exceeded

the threshold for sufficient time. Amplitude and time hysteresis are used to reduce chattering in marginal

conditions.

See Section 1.7.1 for definitions of Teon and Teoff

Figure 4 Rx Energy Detector Timing

1.5.6

FSK / DTMF Modulator

When bit 7 of the TX TONES Register is set to ‘0’ then this block generates FSK signals as determined by

bits 0 and 1 of the FSK MODE Register and the Tx data bits from the UART block as shown in the tables

below:

V.23 mode (bit 7 of SETUP register = ‘0’):

FSK MODE Reg

FSK / DTMF Modulator block output

FSK Signal Frequency

Bit 1

Bit 0

(Bit 7 of TX TONES = ‘0’)

‘0’ (Space)

‘1’ (Mark)

0

x

-

-

1

0

75bps FSK

450Hz

390Hz

1

1

1200bps FSK

2100Hz

1300Hz