TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-41
V2.0, 2007-07
EBU, V2.0
Note: The Data Write Buffer can only store the data associated with a single write
access. If the EBU receives a PLMB request for a write to the external bus and the
Data Write Buffer is not available (i.e. a previous write is still pending or
underway), the EBU will return an PLMB Retry acknowledge.
For the case where the Data Write Buffer holds the requested read data, a
programmable bypass feature is provided from the Data Write Buffer to the Data Read
Buffer.
13.7
Standard Access Phases
Accesses to asynchronous devices and Burst Flash devices are composed of a number
of standard access phases (according to the type of device and the type of access).
There are six access phases defined:
•
Address Phase AP (mandatory for read and write cycles of both device types)
•
Command Delay Phase CD (optional)
•
Command Phase CP (mandatory for read and write cycles of both device types)
•
Data Hold Phase DH (optional, only applies to write cycles)
•
Recovery Phase RP (optional)
•
Burst Phase BP (mandatory, only applies to burst read cycles)
Throughout the remainder of this document, a short-hand notation is adopted to
represent any clock cycle in any phase. This notation consists of two or three letters
followed by a number. The letters identify the access phase within which the clock cycle
is located (e.g. AP for Address Phase). The number denotes the number of LMBCLK
clock cycles within the phase (i.e. 1 = first, etc.). In the case of delays that can be
extended by external control inputs the lower case letters “e” and “i” are inserted
following the two letter phase identifier to differentiate between internally (“i”) and
externally (“e”) generated delays. For example, AP2 identifies the second clock in the
Address Phase. CPe3 identifies the third clock in the Command Phase which is being
extended by external wait-states.
13.7.1
Address Phase (AP)
The Address Phase is mandatory. It always consists of at least one or more LMBCLK
cycles. The phase can be optionally extended to accommodate slower devices.
At the start of the Address Phase, the EBU:
•
Selects the device to be accessed by asserting the appropriate CSx signal,
•
Issues the address which is to be accessed on the address bus,
•
Asserts the ADV signal low,
•
Asserts the appropriate BCx signals if these are programmed to be asserted with the
CSx signal,