Reading Data From The Eeprom; Writing Data To The Eeprom; Write Protection; Eeprom Interr�Pt - Holtek HT66F20 Manual

Reading Data from the EEPROM

To read data from the EEPROM, the read enable bit, RDEN, in the EEC register must first be set
high to enable the read function. The EEPROM address of the data to be read must then be placed
in the EEA register. If the RD bit in the EEC register is now set high, a read cycle will be initiated.
Setting the RD bit high will not initiate a read operation if the RDEN bit has not been set. When
the read cycle terminates, the RD bit will be automatically cleared to zero, after which the data can
be read from the EED register. The data will remain in the EED register until another read or write
operation is executed. The application program can poll the RD bit to determine when the data is
valid for reading.

Writing Data to the EEPROM

The EEPROM address of the data to be written must first be placed in the EEA register and the data
placed in the EED register. To write data to the EEPROM, the write enable bit, WREN, in the EEC
register must first be set high to enable the write function. After this, the WR bit in the EEC register
must be immediately set high to initiate a write cycle. These two instructions must be executed
consecutively. The global interrupt bit EMI should also first be cleared before implementing any
write operations, and then set again after the write cycle has started. Setting the WR bit high will not
initiate a write cycle if the WREN bit has not been set. As the EEPROM write cycle is controlled
using an internal timer whose operation is asynchronous to microcontroller system clock, a certain
time will elapse before the data will have been written into the EEPROM. Detecting when the write
cycle has finished can be implemented either by polling the WR bit in the EEC register or by using
the EEPROM interrupt. When the write cycle terminates, the WR bit will be automatically cleared
to zero by the microcontroller, informing the user that the data has been written to the EEPROM.
The application program can therefore poll the WR bit to determine when the write cycle has ended.

Write Protection

Protection against inadvertent write operation is provided in several ways. After the device is
powered-on theWrite Enable bit in the control register will be cleared preventing any write
operations. Also at power-on the Bank Pointer, BP, will be reset to zero, which means that Data
Memory Bank 0 will be selected. As the EEPROM control register is located in Bank 1, this adds a
further measure of protection against spurious write operations. During normal program operation,
ensuring that the Write Enable bit in the control register is cleared will safeguard against incorrect
write operations.
EEPROM Interrupt
The EEPROM write or read interrupt is generated when an EEPROM write or read cycle has
ended. The EEPROM interrupt must first be enabled by setting the DEE bit in the relevant interrupt
register. However as the EEPROM is contained within a Multi-function Interrupt, the associated
multi-function interrupt enable bit must also be set. When an EEPROM write cycle ends, the
DEF request flag and its associated multi-function interrupt request flag will both be set. If the
global, EEPROM and Multi-function interrupts are enabled and the stack is not full, a jump to
the associated Multi-function Interrupt vector will take place. When the interrupt is serviced only
the Multi-function interrupt flag will be automatically reset, the EEPROM interrupt flag must be
manually reset by the application program. More details can be obtained in the Interrupt section.
Rev. 2.50
HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
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