Architecture
861
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
External Memory Interface A (EMIFA)
19.2.5.5.2 Asynchronous Write Operations (Select Strobe Mode)
NOTE:
During the entirety of an asynchronous write operation, the EMA_OE pin is driven high.
An asynchronous write is performed when any of the requesters mentioned in
request a
write to memory in the asynchronous bank of the EMIFA. After the request is received, a write operation is
initiated once it becomes the EMIFA's highest priority task, according to the priority scheme detailed in
. In the event that the write request cannot be serviced by a single access cycle to the
external device, multiple access cycles will be performed by the EMIFA until the entire request is fulfilled.
The details of an asynchronous write operation in Select Strobe Mode are described in
. Also,
shows an example timing diagram of a basic write operation.
Table 19-22. Asynchronous Write Operation in Select Strobe Mode
Time Interval
Pin Activity in Select Strobe Mode
Turnaround
period
Once the write operation becomes the highest priority task for the EMIFA, the EMIFA waits for the programmed
number of turn-around cycles before proceeding to the setup period of the operation. The number of wait cycles is
taken directly from the TA field of the asynchronous
n
configuration register (CE
n
CFG). There are two exceptions
to this rule:
• If the current write operation was directly proceeded by another write operation to the same chip select, no
turn-around cycles are inserted.
After the EMIFA has waited for the turnaround cycles to complete, it again checks to make sure that the write
operation is still its highest priority task. If so, the EMIFA proceeds to the setup period of the operation. If it is no
longer the highest priority task, the EMIFA terminates the operation.
Start of the
setup period
The following actions occur at the start of the setup period:
• The setup, strobe, and hold values are set according to the W_SETUP, W_STROBE, and W_HOLD values
in CE
n
CFG.
• The address pins EMA_A and EMA_BA and the data pins EMA_D become valid. The EMA_A and EMA_BA
pins carry the values described in
• The EMA_WE_DQM pins become active as byte enables.
Strobe period
The following actions occur at the start of the strobe period of a write operation:
• EMA_CS[n] (n = 2, 3, 4, or 5) and EMA_WE fall
The following actions occur on the rising edge of the clock which is concurrent with the end of the strobe period:
• EMA_CS[n] (n = 2, 3, 4, or 5) and EMA_WE rise
In
, EMA_WAIT is inactive. If EMA_WAIT is instead activated, the strobe period can be extended by
the external device to give it more time to accept the data.
contains more details on using the
EMA_WAIT pin.
End of the hold
period
At the end of the hold period:
• The address pins EMA_A and EMA_BA become invalid
• The data pins become invalid
• The EMA_WE_DQM pins become invalid
The EMIFA may be required to issue additional write operations to a device with a small data bus width in order
to complete an entire word access. In this case, the EMIFA immediately re-enters the setup period to begin
another operation without incurring the turnaround cycle delay. The setup, strobe, and hold values are not
updated in this case. If the entire word access has been completed, the EMIFA returns to its previous state
unless another asynchronous request has been submitted and is currently the highest priority task. If this is the
case, the EMIFA instead enters directly into the turn-around period for the pending read or write operation.