Cirrus Logic AN312 - Manual

Copyright © Cirrus Logic, Inc. 2008
(All Rights Reserved)
http://www.cirrus.com
CobraNet ® Clocking Modes
1. INTRODUCTION
In most cases, a CobraNet
®
system is used with its default audio clocking mode (0x00). This allows automatic syn-
chronization of audio clocks network wide. However, the clocking circuitry of a CobraNet device can be configured
to operate in a number of ways that are specified by the values written to the syncConductorClock and syncPerform-
erClock Management Interface (MI) variables. These two MI variables are used independently to set the clocking
mode of an interface depending on whether it is operating as a Conductor or a Performer. These variables can be
set by using SNMP or the Host Management Interface (HMI).
Please see the CobraNet Programmer's Reference Manual and the CobraNet Hardware User's Manual for more in-
formation on the variables, SNMP, and the Host Management Interface.
2. IMPORTANT CONCEPTS REGARDING CobraNet CLOCKING
All devices in a CobraNet network must operate within the same clock domain. It is therefore a requirement that,
regardless of clock mode, all audio clocks on the network should be derived from the same master clock to ensure
that they remain synchronized. This is accomplished automatically when using the default clock mode, 0x00, and
when any digital devices attached to the CobraNet interface receive their audio clocks from the CobraNet interface.
• Any CobraNet interface that uses one of the external clocking modes must use external clocks that are syn-
chronous with the network audio clock.
• A digital audio device that does not receive its audio clocks from the CobraNet interface must be connected
to the CobraNet interface using a sample rate converter. For instance, all digital audio devices connected
to CobraNet devices must be in the same audio clock domain as the CobraNet network.
• Any clock mode that uses the external word clock (REFCLK_IN) will not work unless the REFCLK_IN pin is
first enabled by clearing the second bit (bit 1) of the syncBuddyLinkControl MI variable.
3. CORE CLOCK CIRCUITRY OF A CobraNet INTERFACE
The core clock circuitry of a CobraNet interface is depicted in Figures 1 and 2 . Note that not all inputs and outputs
to the clock module are used at the same time in all modes. Operation of the circuitry, and activity on each path, are
dependent on the clock mode selected. These different clocking modes can be used in many ways, depending on
the requirements of a particular application. Common usage of the different modes is described in this document.
The user-available external clock connections are REFCLK, MCLK_IN, MCLK_OUT, FS1, and SCLK. The other
connections are internal to the interface and utilized by the interface's firmware in response to the particular mode
in use.
The five audio clock modes are:
• 0x00 - Internal (Normal Default Mode)
• 0x10 - Internal with External Sample Synchronization
• 0x01 - External Word Clock
• 0x04 - External Master Clock
• 0x14 - External Master Clock with External Sample Synchronization
AN312
MAY ‘ 08
AN312REV2