
9-16
MPC565/MPC566 Reference Manual
MOTOROLA
Bus Operations
read and write operations, this mode should be invoked in order to avoid long term
reliability issues of the data pads.
When PDMCR2[PREDIS_EN] bit is set, the MPC565/MPC566 will discharge the bus
during the address phase of any write cycle prior to the data phase. The data bus will be
discharged from up to 5 V to a level which is suitable to the low voltage drivers. In most
cases, the ORx[EHTR] bit of the relevant memory bank, should be set along with the
PREDIS_EN bit in order to reserve sufficient time for the memory to tri-state the bus before
the bus discharge is initiated. EHTR has a slight performance reduction impact since it adds
a clock gap between some read and write cycles.
NOTE
EHTR also adds one idle clock for two consecutive read cycles
from different memory banks.
9.5.3.1
Operating Conditions
Pre-discharge mode should be enabled in the following cases:
When external devices can charge the data bus to a higher voltage level than 3.1
volts
When one of the following occurs:
— The MPC565/MPC566 uses write accesses to any external memory
— Data show cycles are enabled
— Instruction show cycles are enabled in code compression mode (MPC566 only)
NOTE
In the case of code compression program tracking (3rd case
above), the PREDIS_EN bit should only be set when program
tracking is not required since pre-discharge mode overwrites
the compression show cycles data. The user should not set
PREDIS_EN bit when program tracking is required on
development system, and set PREDIS_EN bit on the
production version. EHTR can always be set to keep the same
system performance during development, and production
phases.
9.5.3.2
Initialization Sequence
Systems that require pre-discharge operation should include the following steps:
Execute boot sequence