ATA5505 Datasheet, PDF (22/294 Pages) ATMEL Corporation – High Performance, Low Power Atmel AVR 8-bit Microcontroller Advanced RISC Architecture

3.3.1
3.3.2
3.3.3
3.3.4
3.3.5
EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access times for the EEPROM are given in Table 3-2. A self-timing function, how-
ever, lets the user software detect when the next byte can be written. If the user code contains
instructions that write the EEPROM, some precautions must be taken. In heavily filtered power
supplies, V_CC is likely to rise or fall slowly on Power-up/down. This causes the device for some
period of time to run at a voltage lower than specified as minimum for the clock frequency
used. See “Preventing EEPROM Corruption” on page 24 for details on how to avoid problems
in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed.
Refer to “Atomic Byte Programming” on page 22 and “Split Byte Programming” on page 22 for
details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction
is executed. When the EEPROM is written, the CPU is halted for two clock cycles before the
next instruction is executed.
Atomic Byte Programming
Using Atomic Byte Programming is the simplest mode. When writing a byte to the EEPROM,
the user must write the address into the EEARL Register and data into EEDR Register. If the
EEPMn bits are zero, writing EEPE (within four cycles after EEMPE is written) will trigger the
erase/write operation. Both the erase and write cycle are done in one operation and the total
programming time is given in Table 1. The EEPE bit remains set until the erase and write
operations are completed. While the device is busy with programming, it is not possible to do
any other EEPROM operations.
Split Byte Programming
It is possible to split the erase and write cycle in two different operations. This may be useful if
the system requires short access time for some limited period of time (typically if the power
supply voltage falls). In order to take advantage of this method, it is required that the locations
to be written have been erased before the write operation. But since the erase and write oper-
ations are split, it is possible to do the erase operations when the system allows doing
time-critical operations (typically after Power-up).
Erase
To erase a byte, the address must be written to EEAR. If the EEPMn bits are 0b01, writing the
EEPE (within four cycles after EEMPE is written) will trigger the erase operation only (pro-
gramming time is given in Table 1). The EEPE bit remains set until the erase operation
completes. While the device is busy programming, it is not possible to do any other EEPROM
operations.
Write
To write a location, the user must write the address into EEAR and the data into EEDR. If the
EEPMn bits are 0b10, writing the EEPE (within four cycles after EEMPE is written) will trigger
the write operation only (programming time is given in Table 1). The EEPE bit remains set until
the write operation completes. If the location to be written has not been erased before write,
the data that is stored must be considered as lost. While the device is busy with programming,
it is not possible to do any other EEPROM operations.
22 Atmel ATA5505 [Preliminary]
9219A–RFID–01/11