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AM79C30AVC

Digital Subscriber Controller™ (DSC™) Circuit

Advanced Micro Devices 

• The LIU receiver is enabled, detects an incoming

signal on the S/T Interface, and achieves activation

as indicated by a transition to state F7. Both the INT

pin and the F7 transition interrupt must be enabled

for Power-Down mode to be exited. If the LIU is en-

abled, it may restart the oscillator so that it can iden-

tify the activity on the interface. If the activity is

determined to be noise, the LIU will stop the oscilla-

tor and continue to monitor the line without an inter-

rupt or returning to Idle mode.

• The IOM-2 Interface is enabled as a clock master

and the SBIN input pin goes Low. This indicates that

a slave device wants to activate the IOM-2 Interface

and communicate with the DSC circuit. Both the INT

pin and the IOM-2 timing request interrupts must be

enabled for Power-Down mode to be exited.

• The IOM-2 Interface is enabled as a clock slave and

the SCLK input pin goes High. This indicates that

the master device is activating the IOM-2 Interface

and the DSC circuit must wake up in order to moni-

tor the data. Both the INT pin and the IOM-2 timing

request interrupts must be enabled for Power-Down

mode to be exited.

If the DSC/IDC circuit is awakened by any condition

other than RESET, the MCLK output will be restored to

its previously programmed frequency, and will not gen-

erate any shortened or spurious output cycles. If the

DSC/IDC circuit is revived by RESET, MCLK will default

to its normal 6.144-MHz rate. The DSC/IDC circuit pro-

vides a minimum of two MCLK cycles prior to activating

the interrupt pin when exiting Power-Down mode.

MCLK Frequency Control

The MCLK frequency selection bits in the INIT register

are unchanged from Revision D. However, additional

MCLK frequencies are available by programming bits in

the INIT2 register. No shortened or spurious clock

pulses that might disrupt the external microprocessor

will result when the MCLK frequency is changed.

In order to reduce the probability of errant software dis-

rupting system operation, the INIT2 register requires

two consecutive writes before the value will be entered

into the register. Note that there will be no MCLK count-

down as is the case for entering Power-Down mode if

INIT2 is programmed to cause MCLK to STOP, and

there will be no shortened or spurious MCLK pulses.

MCLK Clock Speed-up Function

A programmable automatic MCLK speed-up option is

provided that will force a hardware reset of INIT2 bits

3-0, which will cause the MCLK frequency to be re-

stored to the value programmed in the INIT register.

There are two events that will trigger the clock

speed-up function:

1. The DLC receive FIFO threshold has been reached;

or,

2. a second packet begins to be received while data

from a prior packet is still in the receive FIFO.

The second packet case requires provision of an inter-

rupt; see the DLC register section for further informa-

tion. The clock speed-up function allows the user to

program a very slow MCLK frequency using INIT2

when D-channel activity is minimal. If a burst of activity

is seen on the D channel and it exceeds the pro-

grammed threshold of the receive FIFO or threatens to

overrun the receive FIFO status buffers, MCLK will in-

stantly toggle back to the higher frequency pro-

grammed in the INIT register. This eliminates the

latency incurred if an interrupt has to be serviced to

change the clock speed, and allows the overall system

power to be reduced during typical voice connections.

Note that automatic clock speed-up will not function un-

less at least one of the associated interrupts are en-

abled so the processor can be informed that the clock

speed has been altered.

Global Register Functions

INIT Register (INIT) default = 00H

Address = Indirect 21 Hex, Read/Write

Table 1. INIT Register

Bit

7 6 5 4 3 2 1 0 Function

X X X X X X 0 0 Idle mode

X X X X X X 0 1 Active Voice and Data mode

X X X X X X 1 0 Active Data Only mode

X X X X X X 1 1 Power-Down mode

X X X X X 0 X X INT output enabled

X X X X X 1 X X INT output disabled

X X 0 0 0 X X X MCLK frequency = 6.144 MHz

X X 0 0 1 X X X MCLK frequency = 12.288 MHz

X X 0 1 0 X X X MCLK frequency = 3,072 MHz

X X 0 1 1 X X X MCLK frequency = 6.144 MHz

X X 1 0 0 X X X MCLK frequency = 4.096 MHz

X X 1 0 1 X X X MCLK frequency = 6.144 MHz

X X 1 1 0 X X X MCLK frequency = 6.144 MHz

X X 1 1 1 X X X MCLK frequency = 6.144 MHz

X 0 X X X X X X DLC receiver abort disabled

X 1 X X X X X X DLC receiver abort enabled

0 X X X X X X X DLC transmitter abort disabled

1 X X X X X X X DLC transmitter abort enabled

Am79C30A/32A Data Sheet

9

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