Silicon Laboratories SI2493/57/34/15/04 Legacy-Mode Line Voltage and Loop Current Measurement

AN93

3.6. Legacy-Mode Line Voltage and Loop Current Measurement

The 5-bit LVCS register, U79 (LVCS) [4:0], reports line voltage measurements when on-hook and loop current measurements when off-hook.

Using the LVCS bits, the user can determine the following:

When on-hook, detect if a line is connected.

When on-hook, detect if a parallel phone is off-hook.

When off-hook, detect if a parallel phone goes on or off-hook.

Detect if enough loop current is available to operate.

3.7. Billing Tone Detection

Billing tones or metering pulses generated by the central office can cause connection difficulties in modems. The billing tone is typically a 12 kHz or 16 kHz signal and is sometimes used in Germany, Switzerland, and South Africa. Depending on line conditions, the billing tone may be large enough to cause major modem errors. The ISOmodem chipset can provide feedback when a billing tone occurs and when it ends.

Billing tone detection is enabled by setting the BTE bit (U68, bit 2). Billing tones less than 1.1 VPK on the line are filtered out by the low-pass digital filter on the ISOmodem. The ROV bit (U68, bit 1) is set when a line signal is greater than 1.1 VPK, indicating a receive overload condition. The BTD bit is set when a line signal (billing tone) is large enough to excessively reduce the line-derived power supply of the line-side device (Si3018/10). When the BTE bit is set, the dc termination is changed to an 800 Ω dc impedance. This ensures minimum line voltage levels even in the presence of billing tones.

The OVL bit should be polled following billing-tone detection. When the OVL bit returns to 0, indicating that the billing tone has passed, the BTE bit should be written to 0 to return the dc termination to its original state. It takes approximately 1 second to return to normal dc operating conditions. The BTD and ROV bits are sticky and must be written to 0 to be reset. After the BTE, ROV, and BTD bits are cleared, the BTE bit can be set to reenable billing- tone detection.

Certain line events, such as an off-hook event on a parallel phone or a polarity reversal, may trigger the ROV bit or the BTD bit, after which the billing-tone detector must be reset. Look for multiple events before qualifying whether billing tones are actually present.

Although the DAA remains off-hook during a billing-tone event, the received data from the line is corrupted (or a modem disconnect or retrain may occur) in the presence of large billing tones. To receive data in the presence of a billing tone, an external LC filter must be added. A modem manufacturer can provide this filter to users in the form of a dongle that connects on the phone line before the DAA. This keeps the manufacturer from having to include a costly LC filter internal to the modem when it may only be necessary to support a few countries or customers.

Alternatively, when a billing tone is detected, the host software may notify the user that a billing tone has occurred. This notification can be used to prompt the user to contact the telephone company to have the billing tones disabled or purchase an external LC filter.

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