TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-39
V2.0, 2007-07
EBU, V2.0
13.6
PLMB Data Buffering
The EBU contains three data buffers that are used to adapt accesses from the internal
64-bit wide PLMB to the 16-bit or 32-bit wide external bus.
illustrates the
data-buffering scheme.
Figure 13-14 EBU PLMB Data Buffering
13.6.1
Data Read Buffer
This buffer consists of four 64 bit words (8
×
32-bit) and is used whenever a PLMB
master performs a read from a device on the external bus. The buffer is used to
assemble the appropriate data width (e.g. 32-bit or 64-bit) to be delivered to the PLMB
from the appropriate number of 16-bit or 32-bit data fetched from the external bus.
The EBU provides a bypass feature for the case where a PLMB read access is received
which matches a pending write in the Data Write Buffer. In this case the required data is
returned directly from the Data Write Buffer. In all other cases the request is translated
in to the appropriate number of external read accesses. The bypass feature can be
disabled separately for each chip select region via the DLOAD bit in registers
EBU_BUSCON or EBU_EMUBC. In the case of an external device at which reading a
location does not necessarily return the last value written to that location (e.g. hardware
MCA05725
Data Write Buffer
Data Read Buffer
Code Prefetch Buffer
External
Data Bus
(16-bit/32-bit)
Program
Local
Memory
Bus
(64-bit)
