V.22 CALL SETUP WITH THE CMX867 - CML Microcircuits

COMMUNICATION ICs

Application Note

V.22 Call Setup with the

CMX867

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¤2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830147.001

4800 Bethania Station Road, Winston-Salem , NC 27105-1201 USA All Trademarks and service marks are held by their res pec t ive companies.

V.22 Call Setup with the CMX867

This document provides details on the V.22 wireline communications protocol and pointers on how the

handshaking sequence can be achieved with the CMX867 modem IC. Also included is additional information

on the V.14 and V.25 protocols. This document should be used in conjunction with the CMX867 data bulletin.

Detect Unscrambled

ONES in 155±10ms Wait 456±10ms

Scrambled ONES in low band Data

Calling modem

Connect to

line & dial

Wait 765±10ms

Detect

scrambled

ONES in high

band for

270±40ms

75±20ms

Unscrambled ONES in high band

Wait 765±10ms

Scrambled ONES in high band Data

2150

±350ms

Answering modem

Connect to

line after ring

validation

Detect

scrambled

ONES in low

band for

270±40ms

3300ms

±700ms

1800Hz Guard Tone*

Silence Answertone

2100Hz

Fig. 1. V.22 Connect Handshake.

* Guard tones are country dependent

1 V.22 Handshake

Figure 1 above is an adaptation of the V.22 Connect Handshake from Volume VIII - Fascicle VIII.1 of the

Blue Book series published by the ITU (formerly CCITT) November 1988. The actions taken by the

calling and answering modems are illustrated with flow charts and ‘C-BUS’ command strings for

implementation on the CMX867 – see Figures 2 and 3.

1.1 Points to note

• The modem can connect to the line by setting the /RDRV output (CMX867 pin 3) to Vss in order to

drive a relay or semiconductor switch. The on-hook level of /RDRV must be Vdd; otherwise

spurious line seizures could occur on initial power up or reset.

• Dial tone will be present typically 300ms after going off-hook. If dialing without dial tone detection

then a pause of at least 2.7s (max. 8s) should be introduced before dialing commences. If no dial

tone is detected within 5 seconds of going off-hook then the modem should return on-hook.

• DTMF digits should be typically 100ms long with 100ms silence gaps in between.

V.22 Call S etup with the CMX867 2 Application Note

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• Figures 2 and 3 are intended to give general guidance on the use of registers. They do not show

software timeout features that avoid lockup on certain loops. The treatment of interrupts (enabling,

servicing & disabling) is not fully shown since these are options that may be implemented for a

particular design. Guard tones are also not included.

• On the Public Switched Telephone Network (PSTN), the calling modem should transmit on the low

band and receive in the high band.

• Error handling has not been included since this aspect is implementation dependent.

• The detection of in-band energy is indicated by b10 of the Status register ($E6). It is advisable to

check this bit in both call setup and data modes where signals are due to be received since it is

possible for noise to occasionally give false indication of bit patterns on a line which does not in fact

have a true signal.

• The following bits in the Transmit Mode ($E1) and Receive Mode ($E2) registers need to be set

according to the circuit and function requirement:

Tx Mode b11…b9 = Tx gain

Tx Mode b8,b7 = 550Hz/1800Hz Guard Tone enabling

Tx Mode b4…b0 = USART mode and format

Rx Mode b11…b9 = Rx gain

Rx Mode b5…b0 = USART mode and format

• Some points are shown on the drawing in Fig. 1 that relate to circuit switching on the host external

port. For example, the point labeled “Detect 32 consecutive ONES at 2400b/s” is a point at which

the data channel received line signal detector (i.e. Data Carrier Detect pin on RS-232 port, also

referred to as “circuit 109” in the V.22 standard) can be switched.

• Register bit states on the flow charts assume a General Reset ($01) has been issued and the

normal start-up procedure has been followed prior to entering the Calling and the Answering

procedures. Each state during these two procedures lists only the register bit changes that are

required to move from the previous state.

1.1.1 For both Calling and Answering modems:

Power up the modem:

Issue a General Reset Command – C-BUS address = $01 (no data)

If the modems need to be set to auto-answer mode:

Write to the General Control ($E0) register:

b6 = 1 (enable /IRQ output)

b5 = 1 (unmask ring detect IRQ)

The modem is now in powersave mode, but programmed to interrupt the controlling microcontroller

when ringing is detected.

1.1.2 For the Calling modem when ready to dial out:

Take the modem out of powersave mode:

Write to the General Control ($E0) register:

b8, b7 = 11 (power up and reset registers)

Wait 20ms

Write to General Control ($E0) register:

b7 = 0 (remove reset condition)

Proceed as shown in the flow charts for Calling Modem

V.22 Call S etup with the CMX867 3 Application Note

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¤2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830147.001

4800 Bethania Station Road, Winston-Salem , NC 27105-1201 USA All Trademarks and service marks are held by their res pec t ive companies.

1.1.3 For the Auto-Answering Modem when ring interrupt is serviced:

Maintain the CMX867 in powersave mode but monitor the Status register ($E6) to check the ring

detect on/off timing. When valid ringing has been received, proceed as shown in the flow charts for

Answering Modem.

2 V.25 and V.14 Implementation on the CMX867

• V.25 timing has been included within figures 2 and 3. The only point not covered is the optional

transmission of 1300Hz by the calling modem in the period after dialing and before Answer tone

recognition. The cadence is 0.6s ON, 1.75s OFF (typ.). For V.22 this tone is NOT permitted, but the

following details are included for other applications:

TX Mode ($E1) register:

b15..b12 = 0001 (select DTMF / Tones)

b3..b0 = 1001 (1300Hz Calling tone)

• The V.25 automatic answering sequence shall be transmitted from the answer mode modem on

international PSTN connections. The transmission of the V.25 sequence may be omitted on point-to-

point leased circuits, or on national connections on the PSTN where permitted by the administration.

• V.14 specifies stop bit omission / recovery where DTE data speed is different from modem speed and

specifies two difference ranges, +1% to –2.5% (basic) and +2.3% to –2.5% (extended). The CMX867

enables stop bit deletion by providing two Tx Data registers, $E3 for normal transmission and $E4 for

transmission with one stop bit removed from the current format. Consequently, the host can adjust

the removal of stop bits according to V.14 by directing bytes to the appropriate Tx Data register.

• At the receiving end the CMX867 can tolerate missing stop bits if set to basic or extended over-speed

mode:

Rx Mode ($E2) register:

b5…b3 = 101 (for basic range)

or b5…b3 = 100 (for extended range)

or b5…b3 = 110 (for over-speed function disabled)

• The CMX867 passes only eight bit data over the C-BUS to the host controller. The host controller

must read the Status register ($E6) and check for parity, framing, and overflow errors before

reconstituting the character in its original format for onward transmission to the remote DTE.

V.22 Call S etup with the CMX867 4 Application Note

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Set-up Tx & Rx

Mode Registers

Take Off-Hook

Dial digits to line.

Calling

Modem

Error

40s Timer

expired ?

Unscrambled

1 detected for

145ms?

Dial tone

detected ?

Set-up Registers

Tx Mode ($E1)

b15..b12 = 0001 (select DTMF / Tones mode)

b11..b9 = user defined (Tx level)

Rx Mode ($E2)

b15..b12 = 0001 (select DTMF / Tone mode)

b11..b9 = user defined (Rx level)

Take Off-Hook

General Control ($E0)

b9 = 1 (Relay drive pin pulled to Vss)

b5 = 0 (Mask Ring Detect IRQ)

Note: If blind dialing then wait 3 seconds before jumping

to the section below headed "Dial digits to line (DTMF)".

Start a 5 second timer.

Dial Tone Detect (before 5s timer expires)

At this point only the dial tone (or possibly busy tone) should

be heard, (country dependent).

General Control ($E0)

b2 = 1 (unmasks Call Progress Energy detect IRQ

in Status register ($E6))

On interrupt, monitor Status register b10 for Call

Progress Energy. If CP energy detected within 5

secs, check whether dial tone (continuous CP

energy) or busy tone (interrupted CP energy) is

present.

Dial digits to line (DTMF)

Tone cadence of 100ms on and 100ms off (country

dependent) is suggested.

General Control ($E0)

b2 = 0 (mask Call Progress Energy IRQ)

Tx Mode ($E1)

b4 = 1 (for DTMF generation)

b3..b0 = user defined (digits to be dialed)

Tone OFF b4..b0 = 00000 (select No Tone)

Start 40s timer

Detect Answer Tone (before 40s timer expires)

Rx Mode ($E2)

b2..b0 = 010 (Answertone detect mode)

General Control ($E0)

b1 = 1 (unmask 2100Hz Answertone IRQ)

Status ($E6)

On interrupt, monitor bit 7 for 2100Hz

Answertone.

Wait for Answertone to disappear.

Detect Unscrambled 1s for 145ms

The 2225Hz Answertone detector will also trigger on

unscrambled 1s.

General Control ($E0)

b1 = 0 (mask 2100Hz Answertone IRQ)

b0 = 1(unmask 2225Hz Answertone IRQ)

Status ($E6)

On interrupt, monitor bit 6 for 2225Hz

Answertone which shows that unscrambled 1s

are being received. Check that unscrambled 1s

are received for at least 145ms.

Wait for Answertone

to disappear

Error

CONTINUED

Answertone

detected ?

Start 40 second

timer

Start 5s timer

5s timer expired ?

Tone ON

V.25

Figure 2

V.22 Call S etup with the CMX867 5 Application Note

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¤2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830147.001

4800 Bethania Station Road, Winston-Salem , NC 27105-1201 USA All Trademarks and service marks are held by their res pec t ive companies.

Wait for 456ms

Check Status ($E6) bit 6 to confirm that unscrambled 1s are

still being received.

Transmit Scrambled 1s at 1200b/s

Tx Mode ($E1)

b15..b12 = 1100 (select V.22 low band)

b6,b5 = 11 (scrambler and 64 x 1s detect enabled)

b4,b3 = 11 (select synchronous mode)

b2..b0 = 011 (select continous 1s)

Detect scrambled 1s in high band

Rx Mode ($E2)

b15..b12 = 1101 (select V.22 high band)

b7,b6 = 11 (descrambler and 64x1s detect enabled)

General Control ($E0)

b1 = 1 (unmask scrambled 1s IRQ)

b0 = 0 (mask IRQ)

Note that b0 was enabled earlier for 2225Hz Answertone

IRQ, but is disabled here after selecting V.22 high band because no

interrupt is yet required for Rx Data Overflow or Rx Data Ready.

On interrupt, check Status ($E6)

If b10 = 1 (Rx energy detected)

b8,b7 = 11 (scrambled 1s found)

Transmit Scrambled

1s at 1200b/s

Calling

Modem

(2)

Unscrambled 1s

still received ?

Error

Check scrambled 1s are detected for 270ms (nom.) If not then an

error has occured.

Enable Rx USART

NOTE: The use of the autoequalizer can improve performance.

Rx Mode ($E2)

b8 = user defined (autoequalizer setting)

b5..b3 = user defined (Rx USART mode)

b2..b0 = user defined (data bits and parity

settings)

General Control ($E0)

b0 = 1 (unmask Rx Data overflow and Rx

Data Ready IRQ)

b1 = 0 (mask scrambled 1s detection IRQ)

Load Tx Data register ($E3 or $E4) with first byte

Note: Check that Status ($E6) bit 12 = 1 (Tx Data Ready) before

loading the data.

Wait for 765ms

Start data transmission

Tx Mode ($E1)

b4,b3 = user defined (Tx USART settings)

b2..b0 = user defined (data and stop bit settings)

General Control ($E0)

b3 = 1 (unmask TX Data underflow and Tx

Data Ready IRQ)

Scrambled

1s in high band for

270ms ?

Wait 765ms

Start data

transmission

Enable Rx USART

Load first data byte

to be transmitted

Error

Scrambled

1s detected in high

band ?

Wait 456ms

V.22 Call S etup with the CMX867 6 Application Note

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¤2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830147.001

4800 Bethania Station Road, Winston-Salem , NC 27105-1201 USA All Trademarks and service marks are held by their res pec t ive companies.

Power Up on Ring

qualification

Answering

Modem On Ring Interrupt

A ring interrupt should wake the host controller to perform

a ring qualification routine and power up the CMX867.

Power up

General Control ($E0)

b8 = 1(Device powered normally)

b7 = 1(Reset internal circuitry)

Wait 20ms

Initialize Registers

General Control ($E0)

b7 = 0 (Normal operation)

Tx Mode ($E1)

b15..12 = 0001 (select DTMF/Tones mode)

b11..9 = user defined (Tx level)

Rx Mode ($E2)

b15..12 = 1100 (select V.22 low band)

b11..9 = user defined (Rx level)

b7,b6 = 11 (descrambler enabled)

Take Off-Hook

General Control ($E0)

b9 = 1 (Relay drive pin pulled to Vss)

Wait for 2150ms.

Transmit Answertone

Tx Mode ($E1)

b3..0 = 1010 (2100Hz Answertone)

or b3..0 = 1011 (2225Hz Answertone)

Wait for 3300ms

Turn off Answertone

Tx Mode ($E1)

b3..0 = 0000 (No Tone)

Wait for 75ms

Transmit Unscrambled 1s

Tx Mode ($E1)

b15..12 = 1101 (select V.22 high band)

b8,b7 = user defined (Tx guard tone)

b4,b3 = 11 (select synchronous mode)

b2..b0 = 011 (select continuous 1s)

Detect scrambled 1s

General Control ($E0)

b1 = 1 (unmask scrambled 1s detect IRQ)

On interrupt, check Status ($E6)

If b10 = 1 (Rx energy detected)

b8,b7 = 11 (scrambled 1s found)

Wait 20ms

Take Off-Hook

Wait 3300ms

Wait 2150ms

CONTINUED

Scrambled

1s detected in low

band?

Transmit

unscrambled 1s

Transmit

Answertone

Initialize Registers

Turn off Answertone

Wait 75ms

V.25

Figure 3

V.22 Call S etup with the CMX867 7 Application Note

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¤2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830147.001

4800 Bethania Station Road, Winston-Salem , NC 27105-1201 USA All Trademarks and service marks are held by their res pec t ive companies.

Check that scrambled 1s are detected for 270ms (nom.)

If not then an error has occured.

Enable Rx USART

NOTE: The use of the autoequalizer can improve performance.

Rx Mode ($E2)

b8 = user defined (autoequalizer setting)

b5..b3 = user defined (Rx USART mode)

b2..b0 = user defined (data bits and parity settings)

General Control ($E0)

b0 = 1 (unmask Rx Data Overflow & Rx Data Ready IRQ)

b1 = 0 (mask scrambled 1s IRQ)

Transmit scrambled 1s (for 765ms)

Tx Mode ($E1)

b6,b5 = 11 (scrambler enabled)

Load Tx Data register ($E3 or $E4) with first byte

Note: Check that Status ($E6) bit 12 = 1 (Tx Data Ready) before

loading the data.

Wait 765ms

Start data transmission

Tx Mode ($E1)

b4,b3 = user defined (Tx USART settings)

b2..b0 = user defined (data and stop bit settings)

General Control ($E0)

b3 = 1(Unmask TX Data Underflow and Tx Data Ready IRQ)

Enable Rx USART

Load Tx buffer with

first data byte

Transmit scrambled

Wait 765ms

Start data

transmision

Answering

Modem

(2)

Scrambled

1s in low band

for 270ms?

Error