Silicon Laboratories SI2494/39 Isolation Capacitor Interface, Low-Power Modes, Power-Down Mode

AN93

2.3. Isolation Capacitor Interface

The isolation capacitor is a proprietary high-speed interface connecting the modem chip and the DAA chip through a high-voltage isolation barrier provided by two capacitors. It serves three purposes. First, it transfers control signals and transmit data from the modem chip to the DAA chip. Second, it transfers receive and status data from the DAA chip to the modem chip. Finally, it provides power from the modem chip to the DAA chip while the modem is in the on-hook condition. The signaling on this interface is intended for communication between the modem and the DAA chips and cannot be used for any other purpose. It is important to keep the length of the ISOcap™ path as short and direct as possible. The layout guidelines for the pins and components associated with this interface are described in "4.4. Layout Guidelines" on page 49 and must be carefully followed to ensure proper operation and avoid unwanted emissions.

2.4. Low-Power Modes

2.4.1. Power-Down Mode

The Power-Down mode is a lower power state than sleep mode. It is entered immediately upon writing U65 [13] (PDN) = 1. Once in the Power-Down mode, the modem requires a hardware reset via the RESET pin to become active again.

2.4.2. Wake-on-Ring Mode

The ISOmodem can be set to enter a low-power wake-on-ring mode when not connected. Wake-on-ring mode is entered using the command AT&Z. The ISOmodem returns to the active mode when one of the following happens:

There is a 1 to 0 transition on TXD in the UART mode

There is a 1 to 0 transition on CS in the parallel mode

There is a 1 to 0 transition on SSS in the SPI mode

An incoming ring is detected

A parallel telephone is picked up

Line polarity reversal

2.4.3. Sleep Mode

The ISOmodem can be set to enter a low-power sleep mode when not connected and after a period of inactivity determined by the S24 register.

The ISOmodem enters the sleep mode S24 seconds after the last DTE activity, after the transmit FIFO is empty, and after the last data are received from the remote modem. The ISOmodem returns to the active mode when one of the following happens:

There is a 1 to 0 transition on TXD in the UART mode

There is a 1 to 0 transition on CS in the parallel mode

There is a 1 to 0 transition on SSS in the SPI mode

An incoming ring is detected

A parallel telephone is picked up

Line polarity reversal

The delay range for S24 is 1 to 255 seconds. The default setting of S24 = 0 disables the sleep timer and keeps the modem in the normal power mode regardless of activity level.

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