Figure 45. Asynchronous Wait During A Write Access Waveforms; Synchronous Transactions - ST STM32L4x6 Reference Manual

RM0351
1. NWAIT polarity depends on WAITPOL bit setting in FMC_BCRx register.
14.5.5

Synchronous transactions

The memory clock, FMC_CLK, is a submultiple of HCLK. It depends on the value of
CLKDIV and the MWID/ AHB data size, following the formula given below:
Whatever MWID size: 16 or 8-bit, the FMC_CLK divider ratio is always defined by the
programmed CLKDIV value.
Example:
•
If CLKDIV=1, MWID = 16 bits, AHB data size=8 bits, FMC_CLK=HCLK/2.
NOR Flash memories specify a minimum time from NADV assertion to CLK high. To meet
this constraint, the FMC 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 memory 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 FMC does not include the clock cycle when NADV is low in the data latency
count.

Figure 45. Asynchronous wait during a write access waveforms

FMC_CLK divider ratio
DocID024597 Rev 3
Flexible static memory controller (FSMC)
max CLKDIV ( 1 , MWID AHB data size
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