TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-49
V2.0, 2007-07
EBU, V2.0
13.7.7
Multiplication Factor for Access Phase Length
As discussed in the previous sections, the length of each access phase is programmable
as a multiple of the LMBCLK clock period. Since the LMB clock runs significantly faster
than most external devices, some devices may need the EBU to be programmed to use
a large number of LMBCLK clock cycles in a particular phase (or phases). Programming
phases for a large number of LMBCLK clock cycles is supported by provision of a
multiplication factor (or prescaler) scheme. This allows configurations in which one (or
more) of the phases is relatively much longer than the others, and provides a flexible
scheme that allows the user to optimize device accesses by fine tuning the individual
phases for the specific device access characteristics.
A specific multiplier (effectively a clock prescaler) is provided for each chip select
region x. The multiplier values are controlled by the EBU_BUSCONx.CMULT (and
EBU_EMUBC.CMULT) bit fields, providing multiplication factors of 1, 4, 8, 16, and 32.
As some types of device timing requirements for a number of the access phases tend to
be larger than others, these phase length settings are always multiplied by the
appropriate multiplier value. These phases length settings are:
•
Number of Command Phase cycles during read accesses (WAITRDC)
•
Number of Command Phase cycles during write accesses (WAITWRC)
•
Number of Recovery Phase cycles when switching from a read access to a write
access or vice versa (DTARDWR)
•
Number of Recovery Phase cycles when switching between different regions
(DTACS)
Each of the remaining phase length settings can be individually programmed whether or
not they are multiplied by CMULT. These phase length settings are:
•
Number of Address Phase cycles (ADDRC)
•
Number of Command Delay Phase cycles (CMDDELAY)
•
Number of Data Hold Phase cycles (DATAC)
•
Number of Recovery Phase cycles following a read access (RDRECOVC)
•
Number of Recovery Phase cycles following a write access (WRRECOVC)
•
Number of Burst Phases cycles (BURSTC)
The selection (enable/disable control) which of these phase length settings are multiplied
by the multiplication factor is selected by the bits in the multiplication factor bit fields
EBU_BUSCONx.MULTMAP or EBU_EMUBC.MULTMAP.