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ManualsBrandsMICROCHIP TECHNOLOGY ManualsMicrocontrollers, Microprocessors & QuartzesEmbedded microcontroller TQFP 44 (10x10) 8-Bit 16 MHz I/O number 32
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Table Of Contents
20
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
• Bit 1 – EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address and data are cor-
rectly set up, the EEWE bit must be written to one to write the value into the EEPROM. The EEMWE bit must be
written to one before a logical one is written to EEWE, otherwise no EEPROM write takes place. The following pro-
cedure should be followed when writing the EEPROM (the order of steps 3 and 4 is not essential):
1. Wait until EEWE becomes zero.
2. Wait until SPMEN in SPMCR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
5. Write a logical one to the EEMWE bit while writing a zero to EEWE in EECR.
6. Within four clock cycles after setting EEMWE, write a logical one to EEWE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The software must check that the
Flash programming is completed before initiating a new EEPROM write. Step 2 is only relevant if the software con-
tains a Boot Loader allowing the CPU to program the Flash. If the Flash is never being updated by the CPU, step 2
can be omitted. See “Boot Loader Support – Read-While-Write Self-Programming” on page 235 for details about
boot programming.
Caution: An interrupt between step 5 and step 6 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 reGister will be modified, causing the interrupted EEPROM Access to fail. It is recommended to
have the Global Interrupt Flag cleared during all the steps to avoid these problems.
When the write access time has elapsed, the EEWE bit is cleared 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.
• Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal – EERE – is the read strobe to the EEPROM. When the correct address is set
up in the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read. The EEPROM
read access takes one instruction, and the requested data is available immediately. When the EEPROM is read,
the CPU is halted for four cycles before the next instruction is executed.
The user should poll the EEWE bit before starting the read operation. If a write operation is in progress, it is neither
possible to read the EEPROM, nor to change the EEAR Register.
The calibrated Oscillator is used to time the EEPROM accesses. Table 8-1 lists the typical programming time for
EEPROM access from the CPU.
Note: 1. Uses 1MHz clock, independent of CKSEL Fuse setting.
The following code examples show one assembly and one C function for writing to the EEPROM. The examples
assume that interrupts are controlled (for example by disabling interrupts globally) so that no interrupts will occur
during execution of these functions. The examples also assume that no Flash Boot Loader is present in the soft-
ware. If such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.
Table 8-1. EEPROM Programming Time
Symbol
Number of Calibrated RC Oscillator
Cycles
(1)
Typ Programming Time
EEPROM write (from CPU) 8448 8.5ms