What is this SPI Flash memory write speed?

[Onedust]

hello,

I just choose for my project an SPI Flash - M45PE40 – VMW6G. **broken link removed** . It can work with a clock speed of 75MHz, but how much does it take to save a bit of data or to read it? I've looked through the Datasheet and I'm sure that it's somewhere there, but I guess I just don't know how this data is presented. Is it the "PAGE WRITE cycle time (256 bytes)" from the Table 13: AC Specifications (50 MHz) on the page 33 which says that it can take 23ms to save 256 bytes? Or this information is hidden somewhere else?

This applies to DDR I choose and other memory types datasheets I've looked through. You can almost never find information given in Bits/sec. What is the reason? Depends it on the application and not only on particular memory device?

best regards

den

 

[Darkcobra]

In general, Flash reading is possible up to the maximum clock speed, with no other restrictions. Writes always occur in full pages. Even if you only modify a single bit in a page, once you deselect the chip or move to a different page then the full page is rewritten internally, taking 23ms.

Due to this behavior, you might need to put some extra thought into the way you deal with Flash in order to get not only acceptable speed, but lifetime too. Consider a data logger that records one byte per second sequentially to a Flash. The most straight-foward implementation of allowing the page write to occur after each byte is written, would cause 256x more wear on each page than necessary. It would be better to cache bytes in RAM, writing only when a full page is accumulated. In addition, you may want to detect disconnection or loss of the main power source, trigger a flush of the partial cache should this occur, and ensure the regulated supplies can maintain their voltage for the 23ms required to complete the write.

EEPROM works the same way. Not sure about DDR.