PXD20RM Datasheet, PDF (270/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

Table 9-7. Master General Purpose Control Register Descriptions
Name
Bits 0–28
AULB
Description
Setting
Master General Purpose Control Register
Reserved - These bits are reserved for future
expansion. They read as zero and should be written
with zero for upward compatibility.
Arbitrate on Undefined Length Bursts - These bits
are used to select the arbitration policy during
undefined length bursts by this master.
These bits are initialized by hardware reset.
The reset value is 000
NA
000No arbitration will be allowed during an
undefined length burst.
001Arbitration will be allowed at any time
during an undefined length burst.
010Arbitration will be allowed after four
beats of an undefined length burst.
011Arbitration will be allowed after eight
beats of an undefined length burst.
100Arbitration will be allowed after 16
beats of an undefined length burst.
101Reserved
110Reserved
111Reserved
The MGPCR can only be accessed in supervisor mode with 32-bit accesses.
9.3.3 Coherency
Since the content of the registers has a real time effect on the operation of the XBAR it is important for the
user to understand that any register modifications take effect as soon as the register is written. The values
of the registers do not track with slave port related AHB accesses but instead track only with IP bus
accesses.
The exception to this rule are the AULB bits in the MGPCR. These update of these bits is only recognized
when the master on that master port runs an IDLE cycle, even though the IP bus cycle to write them will
have long since terminated successfully. If the AULB bits in the MGPCR are written in between two burst
accesses the new AULB encodings will not take effect until an IDLE cycle has been initiated by the master
on that master port.
9.4 Function
This section describes in more detail the functionality of the XBAR.
9.4.1 Arbitration
The XBAR supports two arbitration schemes; a simple fixed-priority comparison algorithm, and a simple
round-robin fairness algorithm. The arbitration scheme is independently programmable for each slave
port.
9-12
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
Freescale Semiconductor