Audio Codec '97
AC ‘97 Component Specification Revision 2.3 Rev 1.0
20
AC'97 Clock Source
Detection
RESET# Signal Asserted
BIT_CLK Toggling?
12.288MHz signal on BIT_CLK is
being generated externally; codec
uses this signal as the clock.
Yes
No
After RESET# Signal
Deasserted
24.576MHz
crystal present?
24.576MHz Crystal on XTL_IN
and XTL_OUT used by codec to
generate clock on BIT_CLK
Yes
24.576MHz
oscillator present?
No
24.576 MHz signal on XTL_IN
used by codec to generate
12.288MHz clock on BIT_CLK
Yes
14.318MHz
oscillator presnent?
No
14.318 MHz signal on XTL_IN
used by codec to generate
12.288MHz clock on BIT_CLK
Yes
Error condition - no clock
source present
No
Figure 7. Codec Clock Source Detection
2
If, when the RESET# signal has been de-asserted, the codec has not detected a signal on BIT_CLK as defined in the
previous paragraph then the AC ‘97 Codec derives its clock internally from an externally attached 24.576 MHz
crystal
3
or oscillator, or optionally from an external 14.318MHz oscillator
4
, and drives a buffered 12.288MHz clock
to its digital companion Controller over AC-link under the signal name “BIT_CLK”. Clock jitter at the DACs and
ADCs is a fundamental impediment to high quality output, and the internally generated clock will provide AC ‘97
with a clean clock that is independent of the physical proximity of AC ‘97’s companion Digital Controller
(henceforth referred to as “the Controller”).
If BIT_CLK begins toggling while the RESET# signal is still asserted, the clock is being provided by other than the
primary codec, for instance by the controller or by a discrete clock source. In this case, the primary codec must act
as a consumer of the BIT_CLK signal as if it were a secondary codec.
2
This figure is for reference of end states only, and is not intended to imply an actual detection sequence. The
actual flow of the detection is left up to the codec manufacturer.
3
The use of crystal is recommended, but an external oscillator may also be input to AC ‘97 XTAL_IN
4
The mechanism used to determine whether a 14.318MHz or other oscillator is attached to the primary codec is not
currently defined, and is left up to each codec manufacturer. This may be specified in the future revisions of this
specification.