The Watchdog Timer The Watchdog Timer is clocked from a separate On-chip Oscillator that runs at 1 MHz. This is the typical value at VCC = 5V. See “Typical Characteristics” on page 66 for typical values at other VCC levels. By controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted from 16 to 2048 ms, as shown in Table 15. The WDR (Watchdog Reset) instruction resets the Watchdog Timer. Eight different clock cycle periods can be selected to determine the reset period. If the reset period expires without another Watchdog Reset, the ATtiny15L resets and executes from the reset vector. For timing details on the Watchdog Reset, refer to page 17.

The Watchdog Timer Control

Register – WDTCR

34 ATtiny15L

• Bits 7..5 – Res: Reserved Bits

These bits are reserved bits in the ATtiny15L and will always read as zero.

• Bit 4 – WDTOE: Watchdog Turn-off Enable

This bit must be set (one) when the WDE bit is cleared. Otherwise, the Watchdog will not be disabled. Once set, hardware will clear this bit to zero after four clock cycles. Refer to the description of the WDE bit for a Watchdog disable procedure.

• Bit 3 – WDE: Watchdog Enable

When the WDE is set (one), the Watchdog Timer is enabled and if the WDE is cleared (zero), the Watchdog Timer function is disabled. WDE can be cleared only when the WDTOE bit is set (one). To disable an enabled Watchdog Timer, the following procedure must be followed:

1.In the same operation, write a logical “1” to WDTOE and WDE. A logical “1” must be written to WDE even though it is set to one before the disable operation starts.

2.Within the next four clock cycles, write a logical “0” to WDE. This disables the Watchdog.

1187D–12/01

EEPROM Read/Write

Access

The EEPROM access registers are accessible in the I/O space.

The write access time is in the range of 4.6 - 8.2 ms, depending on the frequency of the calibrated RC oscillator. See Table 16 for details. A self-timing function however, lets the user software detect when the next byte can be written. If the user code contains code that writes the EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC 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. CPU operation under these conditions is likely to cause the program counter to perform unintentional jumps and eventually execute the EEPROM write code. To secure EEPROM integrity, the user is advised to use an external under-voltage reset circuit in this case.

In order to prevent unintentional EEPROM writes, a two-state write procedure must be followed. Refer to the description of the EEPROM Control Register for details of this.

When the EEPROM is read or written, the CPU is halted for two clock cycles before the next instruction is executed.

The EEPROM Address

Register – EEAR

• Bit 7, 6 – Res: Reserved Bits

These bits are reserved bits in the ATtiny15L and will always read as zero.

• Bit 5..0 – EEAR5..0: EEPROM Address

The EEPROM Address Register (EEAR) specifies the EEPROM address in the 64 bytes EEPROM space. The EEPROM data bytes are addresses linearly between 0 and 63. The initial value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.

The EEPROM Data Register –

EEDR

The EEPROM Control Register

– EECR

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.

• Bit 7..4 – RES: Reserved Bits

These bits are reserved bits in the ATtiny15L and will always read as zero.

Preventing EEPROM

Corruption

The calibrated oscillator is used to time EEPROM. In Table 16 the typical programming time is listed for EEPROM access from the CPU.

Table 16. Typical EEPROM Programming Times

During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for the CPU and the EEPROM to operate properly. These issues are the same as for board-level systems using the EEPROM and the same design solutions should be applied.

An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Second, the CPU itself can execute instructions incorrectly if the supply voltage for executing instructions is too low.

EEPROM data corruption can easily be avoided by following these design recommendations (one is sufficient):

1.Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by enabling the internal Brown-out Detector (BOD) if

the operating voltage matches the detection level. If not, an external low VCC Reset Protection circuit can be applied.

2.Keep the AVR core in Power-down sleep mode during periods of low VCC. This will prevent the CPU from attempting to decode and execute instructions, effectively protecting the EEPROM registers from unintentional writes.

3.Store constants in Flash memory if the ability to change memory contents from software is not required. Flash memory cannot be updated by the CPU and will not be subject to corruption.