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Method of Synchronizing Data across Asynchronous Time-Division Multiplex Bus

IP.com Disclosure Number: IPCOM000115878D
Original Publication Date: 1995-Jul-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 107K

Publishing Venue

IBM

Related People

Macauley, KB: AUTHOR [+2]

Abstract

The use of a fixed rate Time-Division Multiplex (TDM) serial data bus to interface one or more data conversion devices (e.g., CODECs) operating at sample rates that are different from and asynchronous to the TDM frame rate to a master data processing system (e.g., Digital Signal Processor or DSP) which may also be operating at a different and asynchronous rate is attractive as it allows the use of a single common interface bus within the system. Lossless transmission of asynchronous data over the TDM bus may be accomplished by use of a "valid/invalid frame" protocol. While this will result in data frames being supplied to the master at a rate which is on average the rate of the supplying data conversion device, the time required to transmit specific groups of valid frames will vary (plus/minus) around the average time.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 53% of the total text.

Method of Synchronizing Data across Asynchronous Time-Division Multiplex
Bus

      The use of a fixed rate Time-Division Multiplex (TDM) serial
data bus to interface one or more data conversion devices (e.g.,
CODECs) operating at sample rates that are different from and
asynchronous to the TDM frame rate to a master data processing system
(e.g., Digital Signal Processor or DSP) which may also be operating
at a different and asynchronous rate is attractive as it allows the
use of a single common interface bus within the system.  Lossless
transmission of asynchronous data over the TDM bus may be
accomplished by use of a "valid/invalid frame" protocol.  While this
will result in data frames being supplied to the master at a rate
which is on average the rate of the supplying data conversion device,
the time required to transmit specific groups of valid frames will
vary (plus/minus) around the average time.  This phenomenon can
adversely affect the operation of frame-scheduled real time operating
systems in the master device.

      A simple technique to eliminate these adverse affects is
proposed.

      In one instance of a frame-scheduled real time operating
system, a circular buffer is used to collect incoming data frames
from a data conversion device.  The buffer is some number of samples
long, and interrupts are sent to the master processor when the buffer
is half-full and full.

      These interrupts cause the master to schedule and execute a
number of task routines associated with the processing of the data.
Operating in a real time system, it is imperative that enough time is
allotted for all the tasks to complete prior to the next input.

      If the period between interrupts is fixed (due to a fixed input
data rate) and known, then the operating system can schedule tasks to
execute with assurance that they will have enough time to complete.

      If, however, the period between interrupts varies, as would be
the case with an input stream coming from an asynchronous TDM bus
with a "valid/invalid" frame protocol, then the operating system may
have less time in a specific period than it predicted for task
execution.  Hence, some task or tasks may not finish execution prior
to the next interrupt.

This condition may be eliminated by the following technique:
  1.  Increment the buffer pointer according to the arrival of valid
       data frames from the TDM bus at the TDM sample rate.
  2.  After the issuance of an interrupt, start a time interval
       counter.
  3.  Issue an interrupt only  when the fol...