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Thought this was easy, but I can't find this information on how long / how many cycles a EEPROM read operation on an ATTINY84 takes.

Page 22 of the datasheet has a table with write and erase times: enter image description here

Page 19 shows a code example where first the adress register EEAR is set, then the EERE bit is set and then the data is returned. enter image description here

Besides that I could not find any information on the operation speed.

  • Am I correct in the assumption that the EEPROM read data are instantly available in the EEDR register after EERE bit is set?
  • Does this mean that a EEPROM read is about same fast a storing the data in SRAM?
  • How many cycles does a reap operation take?
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Am I correct in the assumption that the EEPROM read data are instantly available in the EEDR register after EERE bit is set?

Instantly is a bit exaggerated (see below), but let's say immediately, yes. At least, from the programmer's point of view, it is available right after, and the sample code clearly shows so.

Does this mean that a EEPROM read is about same fast a storing the data in SRAM? How many cycles does a reap operation take?

From chapter 5.3.1 of the datasheet you linked:

When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed

And from chapter 5.2.1:

The internal data SRAM access is performed in two clkCPU cycles

Note that for SRAM, read and write timings are usually the same. So they don't really explicitly say it, but these two clock cycles should apply for both.

So a EEPROM read is slower than SRAM access (EEPROM needs you to set EEAR, EECR, and then there is a four cycle penalty, whereas for SRAM, there is only a two cycle penalty), but it is in the same order of magnitude.

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