ARM Technical Support Knowledge Articles

General: Can HTRANS change whilst HREADY is low?

Applies to: AHB

Answer

In general, an AHB master should not change control signals whilst HREADY is low. However it is allowable to change HTRANS in the following conditions:

  • HTRANS = IDLE
    The AHB master is performing internal operations and has not yet committed to a bus transfer. However during the AHB wait states (HREADY low) the master may determine that a bus transfer is required and change HTRANS on the next cycle to NONSEQ.
  • HTRANS = BUSY
    HTRANS is being used to give the master time to complete internal operations, which may be entirely independent of HREADY (i.e. wait states on the AHB). Therefore HTRANS can change on the next cycle to any legal value, i.e. SEQ if the burst is to continue, IDLE if the burst has completed, NONSEQ if a separate burst is to begin.
  • HRESP = SPLIT/RETRY
    As stated in the AHB specification, a master must assert IDLE on HTRANS during the second cycle of the two-cycle SPLIT or RETRY slave response so HTRANS will change value from the first cycle to the second cycle of the response.
  • HRESP = ERROR
    The master is permitted to change HTRANS in reaction to an ERROR response in the same way as in reaction to a SPLIT/RETRY response and cancel any further beats in the current burst (even if HBURST is indicating a defined-length burst). In this case HTRANS changes to IDLE on the second cycle of the response. Alternatively, the master is permitted to continue with the current transfers.

See also:

Article last edited on: 2014-02-06 15:02:15

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