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SRAM Data Memory
Figure 8 shows how the ATmega8 SRAM Memory is organized.
The lower 1120 Data memory locations address the Register File, the I/O Memory, and the internal data SRAM. The first 96 locations address the Register File and I/O Memory, and the next 1024 locations address the internal data SRAM.
The five different addressing modes for the Data memory cover: Direct, Indirect with Displacement, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register File, registers R26 to R31 feature the indirect addressing pointer registers.
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base address given by the Y- or Z-register.
When using register indirect addressing modes with automatic pre-decrement and postincrement, the address registers X, Y and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O Registers, and the 1024 bytes of internal data SRAM in the ATmega8 are all accessible through all these addressing modes. The Register File is described in “General Purpose Register File” on page 10.
Figure 8. Data Memory Map
Register File
R0
R1
R2
...
R29
R30
R31
I/O Registers
$00
$01
$02
...
$3D
$3E
$3F
Data Address Space
$0000
$0001
$0002
...
$001D
$001E
$001F
$0020
$0021
$0022
...
$005D
$005E
$005F
Internal SRAM
$0060
$0061
...
$045E
$045F
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Data Memory Access
Times
This section describes the general access timing concepts for internal memory access. The internal data SRAM access is performed in two clkCPU cycles as described in Figure 9.
Figure 9. On-chip Data SRAM Access Cycles
T1 T2 T3
clkCPU
Address |
Compute Address |
Address Valid |
|
Data |
Write |
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WR |
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Data |
Read |
||
RD |
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Memory Vccess Instruction |
Next Instruction |
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EEPROM Data Memory The ATmega8 contains 512 bytes of data EEPROM memory. It is organized as a separate data space, in which single bytes can be read and written. The EEPROM has an endurance of at least 100,000 write/erase cycles. The access between the EEPROM and the CPU is described bellow, specifying the EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control Register.
“Memory Programming” on page 219 contains a detailed description on EEPROM Programming in SPIor Parallel Programming mode.
EEPROM Read/Write Access The EEPROM Access Registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 1 on page 19. A self-timing function, however, 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, VCC is likely to rise or fall slowly on Powerup/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 21. 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 the description of the EEPROM Control Register 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.
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The EEPROM Address
Register – EEARH and EEARL
Bit |
15 |
14 |
13 |
12 |
11 |
10 |
9 |
8 |
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– |
– |
– |
– |
– |
– |
– |
EEAR8 |
EEARH |
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EEAR7 |
EEAR6 |
EEAR5 |
EEAR4 |
EEAR3 |
EEAR2 |
EEAR1 |
EEAR0 |
EEARL |
|||||
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
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Read/Write |
R |
R |
R |
R |
R |
R |
R |
R/W |
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R/W |
R/W |
R/W |
R/W |
R/W |
R/W |
R/W |
R/W |
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Initial Value |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
X |
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X |
X |
X |
X |
X |
X |
X |
X |
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• Bits 15..9 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 and will always read as zero.
• Bits 8..0 – EEAR8..0: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL – specify the EEPROM address in the 512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and 511. The initial value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.
The EEPROM Data Register –
EEDR
Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
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 |
• 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.
The EEPROM Control Register
– EECR
Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
– |
– |
– |
– |
EERIE |
EEMWE |
EEWE |
EERE |
EECR |
|
Read/Write |
R |
R |
R |
R |
R/W |
R/W |
R/W |
R/W |
|
Initial Value |
0 |
0 |
0 |
0 |
0 |
0 |
X |
0 |
• Bits 7..4 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 and will always read as zero.
• Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a constant interrupt when EEWE is cleared.
• 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, setting EEWE within four clock cycles will write data to the EEPROM at the selected address If EEMWE is zero, setting EEWE will have no effect. When EEMWE has been written to one by software, hardware clears the bit to zero after four clock cycles. See the description of the EEWE bit for an 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 written to one to write the
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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 procedure 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 contains 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 206 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 1 lists the typical programming time for EEPROM access from the CPU.
Table 1. EEPROM Programming Time
Number of Calibrated RC |
||
Symbol |
Oscillator Cycles(1) |
Typ Programming Time |
EEPROM Write (from CPU) |
8448 |
8.5 ms |
Note: 1. Uses 1 MHz clock, independent of CKSEL Fuse settings.
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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 software. If such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write sbic EECR,EEWE
rjmp EEPROM_write
; Set up address (r18:r17) in address register out EEARH, r18
out EEARL, r17
; Write data (r16) to data register out EEDR,r16
; Write logical one to EEMWE sbi EECR,EEMWE
; Start eeprom write by setting EEWE sbi EECR,EEWE
ret
C Code Example
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */ while(EECR & (1<<EEWE))
;
/* Set up address and data registers */ EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMWE */
EECR |= (1<<EEMWE);
/* Start eeprom write by setting EEWE */
EECR |= (1<<EEWE);
}
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The next code examples show assembly and C functions for reading the EEPROM. The examples assume that interrupts are controlled so that no interrupts will occur during execution of these functions.
Assembly Code Example
EEPROM_read:
; Wait for completion of previous write sbic EECR,EEWE
rjmp EEPROM_read
; Set up address (r18:r17) in address register out EEARH, r18
out EEARL, r17
; Start eeprom read by writing EERE sbi EECR,EERE
; Read data from data register in r16,EEDR
ret
C Code Example
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */ while(EECR & (1<<EEWE))
;
/* Set up address register */ EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from data register */ return EEDR;
}
EEPROM Write during Power- When entering Power-down sleep mode while an EEPROM write operation is active, the
down Sleep Mode |
EEPROM write operation will continue, and will complete before the Write Access time |
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has passed. However, when the write operation is completed, the Oscillator continues |
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running, and as a consequence, the device does not enter Power-down entirely. It is |
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therefore recommended to verify that the EEPROM write operation is completed before |
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entering Power-down. |
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Preventing EEPROM |
During periods of low VCC, the EEPROM data can be corrupted because the supply volt- |
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Corruption |
age is too low for the CPU and the EEPROM to operate properly. These issues are the |
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same as for board level systems using EEPROM, and the same design solutions should |
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be applied. |
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An EEPROM data corruption can be caused by two situations when the voltage is too |
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low. First, a regular write sequence to the EEPROM requires a minimum voltage to |
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operate correctly. Second, the CPU itself can execute instructions incorrectly, if the sup- |
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ply voltage is too low. |
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EEPROM data corruption can easily be avoided by following this design |
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recommendation: |
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21 |
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2486O–AVR–10/04 |
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