LAN8804DSLASHZMX datasheet(54/89 Pages) MICROCHIP

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LAN8804D

DS00003591K-page 54

5.19.3

SOFTWARE POWER-DOWN MODE

The device supports a Software Power-Down (SPD) mode. This mode is used to power down the device when it is not

in use after power-up. SPD mode is enabled by writing a one to the Power Down bit in the Basic Control register. The

device may also be placed into software power-down by default by setting the MODE_SEL[4:0] configuration straps to

‘0100x’. The device exits the SPD state after a zero is written to the Power Down bit.

In the SPD state, the device disables most internal functions. During SPD, the crystal oscillator and PLL are enabled

and the internal (125 MHz and 250 MHz) clocks are gated. The standard registers (0 through 31) and the MII Manage-

ment Interface operate using the crystal clock.

Previous register settings are maintained during and following the removal of SPD.

APPLICATION NOTE: The internal (125 MHz and 250 MHz) clock gating maybe overridden by setting the

spd_clock_gate_override bit in the Digital Debug Control 2 register at the cost of increased

power.

The following remain operational during SPD:

• MII Management Interface

- Only access to the standard registers (0 through 31) is supported.

- Access to MMD address spaces other than MMD Address Space 1 is possible if the spd_clock_gate_override

bit is set.

- Access to MMD Address Space 1 is not possible.

• Voltage Regulator Controller (LDO)

- The LDO has been defeatured and LDO_O should be left disconnected. Microchip recommends disabling the

LDO output by setting the active low LDO enable bit in the Analog Control Register 11. An external source of

1.21V is required for operation.

•PLL

- Normally the PLL is enabled during SPD. It may be disabled by setting the spd_pll_disable bit described

below.

• Crystal Oscillator

- Normally the Crystal Oscillator is enabled during SPD. It may be disabled by setting the enXTALb bit

described below.

• Bandgap

- This is always enabled.

• Internal Slow Oscillator

- This is always enabled.

The following are normally disabled during SPD:

• ADC/PGA/TX/common bias circuits

•DLL

• TX and RX clocks

- If the above mentioned spd_clock_gate_override bit is set, TX and RX clocks would be enabled. They may

alternately be stopped by setting the Isolate (PHY_ISO) bit in the Basic Control register.

5.19.3.1

SPD Extra Power Savings

To achieve a lower power usage, the PLL may be disabled during SPD mode by setting the spd_pll_disable bit in the

Digital Debug Control 2 register prior to entering SPD. The device may also be placed into software power-down with

the PLL disabled by default by setting the MODE_SEL[4:0] configuration straps to ‘01001’.

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