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ST9 USER GUIDE

4.5.3 EEPROM Serial Interface Example using I²C

An I²C Serial EEPROM is a very convenient device that stores a few bytes for at least 10 years

in a secure manner. The serial I²C access uses only two wires, which allows the memory to fit

in an 8-pin DIL package. The serial access requires a serial transmission protocol, which

makes the access more difficult than with a parallel-bus EEPROM device. This, however, is

not a real drawback, for two reasons:

– The serial synchronous transmission is very easy to implement, and is not time-critical

– Using a serial protocol makes it impossible (or very unlikely) that you will overwrite or erase

data by mistake, which ensures a high safety level for the application

Thus, for applications that can manage with a small amount of permanent data, a serial EE-

PROM is the best choice. Examples include storing the reading of a counter or a recording

meter, storing configuration parameters or calibration values. The application described here

uses a 24C08 memory chip from STMicroelectronics, that takes advantage of the SPI to ex-

change data with minimum software overhead.

4.5.3.1 Additional Clock Timing

Compared to I²C timing, SPI timing needs additional software to manage the following:

– The start condition

– The ninth bit for acknowledgement

– The stop condition

These three timing differences are implemented by changing the input/output port data and

configuration registers, to pull the level on the SDA and SCL pins up or down.

4.5.3.2 EEPROM I²C Protocol

To connect more than one device to an I²C interface, each device needs an address to be se-

lected. The EEPROM 24C08, used for our example, has an address equal to A0h.

Following a start condition, the bus master (SPI) must output the address of the EEPROM it is

accessing. The most significant four bits are the device type identifier (1010). The following bit

identifies the specific memory location on the bus (it is matched to the chip enable signal).

Once the 6th and 7th bits are sent, select the block number and the 8th bit sent is the read or

write bit (1 for read, 0 for write). After the address has been sent, ten data bytes can be sent

to the EEPROM for writing, and all the data bytes can be read successively from the EEPROM

for verification.

ST9 数据表(PDF) 91 Page - STMicroelectronics