
MPC509
USER’S MANUAL
SYSTEM INTERFACE UNIT
Rev. 15 June 98
MOTOROLA
5-59
case, the second region can hold off its data until its OE is asserted. The chip-
select module can pipeline the second access (if a read) with the first access
after it has received the AACK signal for the first access. The chip-select logic
asserts the CE of the second access while the data phase of the first access is
still in progress, if the second access is issued before the first access is com-
pleted.
3. If the first access is to a region that is not under chip-select control (external
glue logic generates all control and handshake signals for the region, as for a
DRAM controller, for example), and the second access is to a region that is un-
der chip-select control, the chip-select module does notpipeline the second ac-
cess with the first.
4. If the first access is to a region under chip-select control and the second access
is a read access to a region that is not under chip-select control, the external
glue logic designer must decide whether to pipeline the second access with the
first. The decision depends on system requirements and on the interface type
of the region that is not under chip-select control.
5. If the first access is a burst read access to a burstable region and the second is
a read access to another region, the chip-select module pipelines the second
read if the second access is to a region with an interface type that is pipeline-
able and can hold off its data. If ITYPE = 8 for the second region, the chip-select
module does not pipeline the second access with the first.
6. If the first access is to a synchronous region, and the second access is to an
asynchronous region, the chip-select module does notpipeline the accesses.
7. If the first access is to an asynchronous region, the chip-select module does not
pipeline the second access with the first, since both the external address and
data bus must be available for the first access until it is completed. If the first
region requires an extra clock to turn off its buffer, the chip-select logic allows
an extra clock for the region.
5.5.16 Chip-Select Timing Diagrams
The diagrams in this section show the different device interfaces that the chip-select
module supports. Where applicable, the diagrams indicate how the various signals
(address, data, and chip-select signals) are correlated.
CAUTION
The user must not assume that CE is always asserted simulta-
neously with TS. Depending on the state of the pipeline (which
depends on the interface types of the devices being accessed), the
chip-select unit may delay asserting CE until one or more clock
cycles after TS is asserted.
5.5.16.1 Asynchronous Interface
An external device with an asynchronous interface requires the address and the chip
select signals (CE, OE, and WE) to be valid until the end of the access. The next
access to the same device must wait for the previous access to complete. No overlap