Flash program memory and data EEPROM (FLASH) RM0367
78/1043 RM0367 Rev 7
Figure 5 shows the timing to fetch and execute instructions from the NVM with 0 wait states
(a) and 1 wait state (b) when the prefetch is enabled. The read executed by the prefetch
appears in green.
Read as data and pre-read
A data read from the memory interface, corresponds to any read operation that is not a
fetch. The master reads operation constants and parameters as data. All reads done by
DMA (to copy from one address to another) are read as data. No check is done on the
location of the data read (can be in every area of the NVM).
At reset, (DISAB_BUF = 0, PRFTEN = 0, PRE_READ = 0), the memory interface uses 2
buffers organized in one group to store the last two values read as data.
In some particular cases (for example when the DMA is reading a lot of consecutive words
in the NVM), it can be useful to enable the pre-read (PRE_READ = 1 with DISAB_BUF = 0).
The pre-read works exactly like the prefetch: it is a speculative reading at the last data
address increased by 4 (one word). With this configuration, one buffer of data is moved to a
new group to store the pre-read value, while the second buffer continues to store the last
value read. For a prefetch, the pre-read value is copied in the last read value if the master
requests it, or is lost if the master requests a different address.
The pre-read has a lower priority than a normal read or a prefetch operation: this means that
it will be launched only when no other type of read is ongoing. Pay attention to the fact that
a pre-read used in a wrong situation can be harmful: in a code where a data read is not
done linearly, reducing the number of buffers (from 2 to 1) used for the last read value can
increase the number of accesses to the NVM (and the time to read the value). Moreover,
this can generate a delay on prefetch. An example of this situation is the code Dhrystone,
whose results are shown in the corresponding section.
As for a prefetch operation, the user must select the right moment to enable and disable the
pre-read.
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