Architecture
868
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
External Memory Interface A (EMIFA)
19.2.5.6.7 NAND Flash Status Register (NANDFSR)
The NAND Flash status register (NANDFSR) indicates the raw status of the EMA_WAIT pin while in
NAND Flash Mode. The EMA_WAIT pin should be connected to the NAND Flash device's R/B signal, so
that it indicates whether or not the NAND Flash device is busy. During a read, the R/B signal will transition
and remain low while the NAND Flash retrieves the data requested. Once the R/B signal transitions high,
the requested data is ready and should be read by the EMIFA. During a write/program operation, the R/B
signal transitions and remains low while the NAND Flash is programming the Flash with the data it has
received from the EMIFA. Once the R/B signal transitions high, the data has been written to the Flash and
the next phase of the transaction may be performed. From this explanation, you can see that the NAND
Flash status register is useful to the software for indicating the status of the NAND Flash device and
determining when to proceed to the next phase of a NAND Flash operation.
When a rising edge occurs on the EMA_WAIT pin, the EMIFA sets the WR (Wait Rise) bit in the EMIFA
interrupt raw register (INTRAW). Therefore, the EMIFA Wait Rise interrupt may be used to indicate the
status of the NAND Flash device. The WP
n
bit in the asynchronous wait cycle configuration register
(AWCC) does not affect the NAND Flash status register (NANDFSR) or the WR bit in INTRAW. See
for more a detailed description of the wait rise interrupt.
19.2.5.6.8 Interfacing to a Non-CE Don't Care NAND Flash
As explained in
, the EMIFA does not support NAND Flash devices that require the chip
select signal to remain low during the t
R
time for a read. One way to work around this limitation is to use a
GPIO pin to drive the CE signal of the NAND Flash device. If this work around is implemented, software
will configure the selected GPIO to be low, then begin the NAND Flash operation, starting with the
command phase. Once the NAND Flash operation has completed the software can then configure the
selected GPIO to be high.
19.2.5.7 Extended Wait Mode and the EMA_WAIT Pin
The EMIFA supports the Extend Wait Mode. This is a mode in which the external asynchronous device
may assert control over the length of the strobe period. The Extended Wait Mode can be entered by
setting the EW bit in the asynchronous
n
configuration register (CE
n
CFG) (
n
= 2, 3, 4, or 5). When this bit
is set, the EMIFA monitors the EMA_WAIT pin to determine if the attached device wishes to extend the
strobe period of the current access cycle beyond the programmed number of clock cycles.
When the EMIFA detects that the EMA_WAIT pin has been asserted, it will begin inserting extra strobe
cycles into the operation until the EMA_WAIT pin is deactivated by the external device. The EMIFA will
then return to the last cycle of the programmed strobe period and the operation will proceed as usual from
this point. Please refer to the device data manual for details on the timing requirements of the EMA_WAIT
signal.
The EMA_WAIT pin cannot be used to extend the strobe period indefinitely. The programmable
MAX_EXT_WAIT field in the asynchronous wait cycle configuration register (AWCC) determines the
maximum number of EMA_CLK cycles the strobe period may be extended beyond the programmed
length. When the counter expires, the EMIFA proceeds to the hold period of the operation regardless of
the state of the EMA_WAIT pin. The EMIFA can also generate an interrupt upon expiration of this counter.
See
for details on enabling this interrupt.
For the EMIFA to function properly in the Extended Wait mode, the WP
n
bit of AWCC must be
programmed to match the polarity of the EMA_WAIT pin. In its reset state of 1, the EMIFA will insert wait
cycles when the EMA_WAIT pin is sampled high. When set to 0, the EMIFA will insert wait cycles only
when EMA_WAIT is sampled low. This programmability allows for a glueless connection to larger variety
of asynchronous devices.
Finally, a restriction is placed on the strobe period timing parameters when operating in Extended Wait
mode. Specifically, the sum of the W_SETUP and W_STROBE fields must be greater than 4, and the sum
of the R_SETUP and R_STROBE fields must be greater than 4 for the EMIFA to recognize the
EMA_WAIT pin has been asserted. The W_SETUP, W_STROBE, R_SETUP, and R_STROBE fields are
in CE
n
CFG.