78Q2120C Teridian Semiconductor Corp. (TDK Semiconductor), 78Q2120C Datasheet - Page 2

• Manufacturer Part Number: 78Q2120C
• Description: 10/100 Ethernet PHYS
• Manufacturer: Teridian Semiconductor Corp. (TDK Semiconductor)

FUNCTIONAL DESCRIPTION
GENERAL
Power Management
The 78Q2120C has three power saving modes:
Chip power-down is activated by setting the PWRDN
bit in the MII register (MR0.11) or pulling high the
PWRDN pin.
mode, all on-chip circuitry is shut off, and the device
consumes minimum power. While in power-down
state, the 78Q2120C still responds to management
transactions.
Receive power management (RXCC mode) is
activated by setting the RXCC bit in the MII register
(MR16.0). In this mode of operation, the adaptive
equalizer, the clock recovery phase lock loop (PLL),
and all other receive circuitry will be powered down
when no valid MLT-3 signal is present at the UTP
receive line interface. As soon as a valid signal is
detected, all circuits will automatically be powered
up to resume normal operation. During this mode of
operation, RX_CLK will be inactive when there is no
data being received. Note that the RXCC mode is
not supported during 10BASE-T operation.
Transmit high impedance mode is activated by
setting the TXHIM bit in the MII register (MR16.12).
In this mode of operation, the transmit UTP drivers
are in a high impedance state and TX_CLK is tri-
stated.
TX_CLK.
operational. The default state of MR16.12 is a logic
low for disabling the transmit high impedance mode.
The transmitter is fully functional when MR16.12 is
cleared.
Analog Biasing and Supply Regulation
The 78Q2120C requires no external component to
generate on-chip bias voltages and currents. High
accuracy is maintained through a closed-loop
trimmed biasing network.
On-chip digital logic runs off an internal voltage
regulator.
required to power-up the device.
regulator is not affected by power-down mode.
© 2003 TDK Semiconductor Corporation, Proprietary and Confidential
•
•
•
Chip Power-Down
Receive Power Management
Transmit High Impedance Mode
A weak internal pull-up is enabled on
The
Hence only a single Vcc supply is
When the chip is in power-down
receive
circuitry
remains
The on-chip
fully
- 2 -
Clock Selection
The 78Q2120C will use the on-chip crystal oscillator
as the clock source if the CKIN pin is tied low. In this
mode of operation, a 25MHz crystal should be
connected between the XTLP and XTLN pins.
Alternatively, an external 25MHz clock input can be
connected to the CKIN pin. The chip senses activity
on the CKIN pin, and will automatically configure itself
to use the external clock, if present. In this mode of
operation, a crystal is not required and the XTLP and
XTLN pins should be left floating or connected
together.
Transmit Clock Generation
The
synthesizer to generate the transmit clock. In
100BASE-TX operation, the synthesizer multiplies the
reference clock by 5 to obtain the internal 125MHz
serial transmit clock.
generates an internal 20MHz transmit clock by
multiplying the reference 25MHz clock by 4/5. The
synthesizer references either the local 25 MHz crystal
oscillator, or the externally applied clock, depending
on the selected mode of operation.
Receive Signal Qualification
The integrated signal qualifier has separate squelch
and unsquelch thresholds, and includes a built-in timer
to ensure fast and accurate signal detection and line
noise rejection. Upon detection of two or more valid
10BASE-T or 100BASE-TX pulses on the line receive
port, the PASS signal, indicating the presence of valid
receive signal or data, will be asserted. When PASS is
asserted, the signal detect threshold is lowered by
about 60%, and all adaptive circuits are released from
their initial states and allowed to lock onto the incoming
data. In 100BASE-TX operation, PASS will be de-
asserted when no signal is presented for a period of
about 1.2us. In 10BASE-T operation, PASS will be de-
asserted whenever no Manchester data is received. In
either case, the signal detect threshold will return to the
squelched level whenever the PASS indication is de-
asserted. The PASS signal is also used to control the
operation of the clock/data recovery circuit to assure
fast acquisition.
Receive Clock Recovery
In 100BASE-TX mode, the 125MHz receive clock is
extracted using a digital DLL-based loop. When no
receive signal is present, the DLL is directed to lock
onto the 125MHz transmit serial clock. When PASS is
asserted, the DLL will use the received MLT-3 signal
as the clock reference. The recovered clock is used to
10/100BASE-TX Transceiver
transmitter
uses
In 10BASE-T mode, it
an
on-chip
78Q2120C
frequency
Rev_1.1