TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-76
V2.0, 2007-07
EBU, V2.0
13.9.11.1 Wait for Page Load Mode
Note: The description in this section assumes a low active WAIT input (WAITINV = 0).
In Wait for Page Load Mode, the WAIT input being asserted low during a burst access
causes the EBU to complete the current Burst Phase and then to perform an additional
Burst Phase (if the previous Burst Phase was not the last of the current burst access
cycle). Following this, the next Burst Phase (again if this Burst Phase required) will not
be started until the WAIT input is de-asserted. Additionally, the start of the next Burst
Phase is also synchronized to the next rising edge of the BFCLKO signal to ensure that
the EBU continues to sample data from the device at the correct time with respect to
BFCLKO. This mode allows, for example, an Intel Burst Flash device to temporarily
suspend the burst sequence in order to perform a page load when a page boundary is
crossed during the access.
The Wait for Page Load Mode supports the use of Intel Burst Flash devices (and
compatibles) configured for Early Wait Generation Mode (EBU_BUSCON.WAIT = 01
B
)
and standard wait generation (EBU_BUSCON.WAIT = 10
B
).
In operation, the Burst Flash controller loads a counter with the required number of
samples at the start of each burst. At the end of each Burst Phase, the Burst Flash
controller samples the WAIT input and the data bus. If WAIT is inactive, the sample is
valid, the sample counter is decremented, and the sampled data is passed to the data
path of the EBU. This synchronous sampling means that the validity of the sample can
not be determined until the clock cycle after the end of the burst phase. The Burst Flash
controller in the EBU will therefore overrun and generate extra burst phases until the
sample counter is decremented to zero. Extra data samples returned after the sample
counter is zero will be discarded.
This mode of operation is compatible with the use of clock feedback as, with feedback
enabled, WAIT is fed through the same re-synchronization signals as the data bus. The
Table 13-20 WAIT Mode Selection
EBU_BUSCON.
WAIT
EBU_BFCON.
WAITFUNC
Selected WAIT Mode
00
B
–
WAIT function disabled
01
B
0
Asynchronous Wait for Page Load
1
Asynchronous Terminate and Start New Burst
10
B
0
Synchronous Wait for Page Load
1
Synchronous Terminate and Start New Burst
11
B
0
Synchronous Data Handshake
1
WAIT function disabled