Flexible static memory controller (FSMC)
RM0090
1558/1731
DocID018909 Rev 11
36.5.5 Synchronous
transactions
The memory clock, CLK, is a submultiple of HCLK according to the value of parameter
CLKDIV.
NOR Flash memories specify a minimum time from NADV assertion to CLK high. To meet
this constraint, the FSMC does not issue the clock to the memory during the first internal
clock cycle of the synchronous access (before NADV assertion). This guarantees that the
rising edge of the memory clock occurs
in the middle
of the NADV low pulse.
Data latency versus NOR Flash latency
The data latency is the number of cycles to wait before sampling the data. The DATLAT
value must be consistent with the latency value specified in the NOR Flash configuration
register. The FSMC does not include the clock cycle when NADV is low in the data latency
count.
Caution:
Some NOR Flash memories include the NADV Low cycle in the data latency count, so the
exact relation between the NOR Flash latency and the FMSC DATLAT parameter can be
either of:
•
NOR Flash latency = ( 2) CLK clock cycles
•
NOR Flash latency = ( 3) CLK clock cycles
Some recent memories assert NWAIT during the latency phase. In such cases DATLAT can
be set to its minimum value. As a result, the FSMC samples the data and waits long enough
to evaluate if the data are valid. Thus the FSMC detects when the memory exits latency and
real data are taken.
Other memories do not assert NWAIT during latency. In this case the latency must be set
correctly for both the FSMC and the memory, otherwise invalid data are mistaken for good
data, or valid data are lost in the initial phase of the memory access.
Single-burst transfer
When the selected bank is configured in burst mode for synchronous accesses, if for
example an AHB single-burst transaction is requested on 16-bit memories, the FSMC
performs a burst transaction of length 1 (if the AHB transfer is 16-bit), or length 2 (if the AHB
transfer is 32-bit) and de-assert the chip select signal when the last data is strobed.
Clearly, such a transfer is not the most efficient in terms of cycles (compared to an
asynchronous read). Nevertheless, a random asynchronous access would first require to re-
program the memory access mode, which would altogether last longer.
Cross boundary page for Cellular RAM 1.5
Cellular RAM 1.5 does not allow burst access to cross the page boundary. The FSMC
controller allows to split automatically the burst access when the memory page size is
reached by configuring the CPSIZE bits in the FSMC_BCR1 register following the memory
page size.
Wait management
For synchronous NOR Flash memories, NWAIT is evaluated after the programmed latency
period, (2) CLK clock cycles.
