AT90S4414/8515
40
EEPROM Data Register - EEDR
•
Bits 7..0 - EEDR7..0: EEPROM Data
For the EEPROM write operation, the EEDR register contains the data to be written to the EEPROM in the address given
by the EEAR register. For the EEPROM read operation, the EEDR contains the data read out from the EEPROM at the
address given by EEAR.
EEPROM Control Register - EECR
•
Bit 7..3 - Res: Reserved bits
These bits are reserved bits in the AT90S4414/8515 and will always read as zero.
•
Bit 2 - EEMWE: EEPROM Master Write Enable
The EEMWE bit determines whether setting EEWE to one causes the EEPROM to be written. When EEMWE is set(one)
setting EEWE will write data to the EEPROM at the selected address If EEMWE is zero, setting EEWE will have no effect.
When EEMWE has been set (one) by software, hardware clears the bit to zero after four clock cycles. See the description
of the EEWE bit for a EEPROM write procedure.
•
Bit 1 - EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address and data are correctly set up,
the EEWE bit must be set to write the value into the EEPROM. The EEMWE bit must be set when the logical one is written
to EEWE, otherwise no EEPROM write takes place. The following procedure should be followed when writing the
EEPROM (the order of steps 2 and 3 is unessential):
1.
Wait until EEWE becomes zero.
2.
Write new EEPROM address to EEARL and EEARH (optional)
3.
Write new EEPROM data to EEDR (optional)
4.
Write a logical one to the EEMWE bit in EECR
5.
Within four clock cycles after setting EEMWE, write a logical one to EEWE.
When the write access time (typically 2.5 ms at V
CC
= 5V or 4 ms at V
CC
= 2.7V) has elapsed, the EEWE bit is cleared
(zero) by hardware. The user software can poll this bit and wait for a zero before writing the next byte. When EEWE has
been set, the CPU is halted for two cycles before the next instruction is executed.
Caution: An interrupt between step 4 and step 5 will make the write cycle fail, since the EEPROM Master Write Enable will
time-out. If an interrupt routine accessing the EEPROM is interrupting another EEPROM access, the EEAR or EEDR regis-
ter will be modified, causing the interrupted EEPROM access to fail. It is recommended to have the global interrupt flag
cleared during the 4 last steps to avoid these problems.
Bit
7
6
5
4
3
2
1
0
$1D ($3D)
MSB
LSB
EEDR
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
$1C ($3C)
-
-
-
-
-
EEMWE
EEWE
EERE
EECR
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial value
0
0
0
0
0
0
0
0