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FM25C640U Rev. B

SPI communication

As mentioned before, serial communication with the EEPROM is

enabled when the /CS pin is held low and the /HOLD pin is held

high. Input data (Instruction Opcodes, Array addresses and Data)

on the SI pin is latched in on the rising edges of SCK clock signal,

starting from the first rising edge after the /CS pin goes low. During

the time the SI data is input into the EEPROM, the SO pin remains

in high impedance state. If the intended instruction is of read

nature (Array read and Status register read), then data from the

EEPROM is driven out actively on the SO pin from every falling

edge of the SCK after the last input data (SI) is latched in. During

the time the SO data is output from the EEPROM, the data on the

SI pin is ignored.

Figure 2 illustrates the above. Refer Figure 1 for

timing information.

HOLD function

An active communication with the EEPROM can be temporarily

suspended by bringing the /HOLD pin low when a EEPROM is

selected (/CS pin should be low) and a serial sequence with the

EEPROM is currently underway. To suspend the communication,

/HOLD pin must be driven low while SCK is low, otherwise the Hold

function will not be invoked until the next SCK high to low

transition. The EEPROM must remain selected during this se-

quence. Transitions on the SCK and SI pins are ignored during the

time the part is suspended and the SO pin will be in high

impedance state. Releasing the /HOLD pin back to high state will

allow the operation to resume from the point it was suspended.

/HOLD pin must be driven high while the SCK pin is low, otherwise

serial communication will not resume until the next SCK high to

low transition. Asserting a low on the /HOLD pin at any time will tri-

state the SO pin.

Figure 3 illustrates Hold timing.

System Configuration

When multiple SPI peripherals (for e.g. EEPROMs) are present on

the bus, the SI, SO and the SCK signals can be tied together.

Figure 4 illustrates a typical system configuration with respect to

/CS, SCK, SI and SO pins.

FIGURE 4. System Configuration

SI

SO

SCK

/CS

DATA OUT (MOSI)

DATA IN (MISO)

SERIAL CLOCK (SPICK)

SS0

SS1

SS2

SS3

SI

SO

SCK

/CS

SI

SO

SCK

/CS

SI

SO

SCK

/CS

SELECTION

CHIP

SPI

MASTER MCU

FM25Cxxx

SPI Modes 0 and 3 (00 and 11)

FM25C640U supports both Mode 0 and Mode 3 of operations.

The difference between Mode 0 and Mode 3 is determined by the

state of the SCK clock signal when a SPI cycle starts (when /CS

is driven low) as well as when the SPI cycle ends (when /CS is

driven high). Under Mode 0 of operation, the SCK signal is held low

both at the start and at the end of a SPI cycle. Under Mode 1 of

operation, the SCK signal is held high both at the start and at the

end of a SPI cycle. However in both of these two modes, the input

data (SI) is sampled (latched in) at the rising edge of the SCK clock

signal and the output data (SO) is driven after the falling edge of

the SCK clock signal. See

Figure 1 and Figure 2.

READ SEQUENCE (READ)

Reading the memory via the serial SPI link requires the following

sequence. The/CS pin is pulled low to select the EEPROM. The

READ opcode is transmitted on the SI pin followed by two bytes of

address, "High byte addr" (A15-A8) and "Low byte addr" (A7-A0).

After this is done, data on the SI pin becomes don't care. The data

(D7-D0) form the address specified is then shifted out on the SO pin.

If only one byte is to be read, the /CS pin can be pulled back to the

high level. It is possible to continue the READ sequence as the byte

address is automatically incremented and data will continue to be

shifted out as clock pulses are continuously applied. When the end

of memory array is reached (last byte location), the address counter

rolls over to the start of memory array (first byte location) allowing

the entire memory to be read in one continuous READ cycle. See

Figure 5. Note that only A12-A0 address bits are treated valid by

FM25C640U while bits A15-A13 are ignored.

FIGURE 5. Read Sequence

;;;;;

CS

SI

SO

Read

Opcode

High Byte

Addr

Low Byte

Addr

Data

(1)

Data

(2)

Data

(n)

READ STATUS REGISTER (RDSR):

The Read Status Register (RDSR) instruction provides read

access to the status register. As mentioned before, of the 8bits of

data, only the LSB 4bits are valid and they indicate Block Protec-

tion information (BP1 and BP0), Write Enable status (WEN) and

Busy/Ready status (/RDY) of the EEPROM. MSB 4bits of are

invalid (Don’t cares) Following is the format of RDSR data:

TABLE 3. Status Register Format

Bit

Bit

Bit

Bit

Bit

Bit

Bit

Bit

7

6

54

32

1

0

X

X

X

X

BP1

BP0

WEN

RDY

Bit3 (BP1) and Bit2 (BP0) together indicate Block write protection

previously set on the EEPROM. Refer Table 2.

Bit1 (WEN) indicates the Write enable status of the EEPROM.

This bit is a read-only bit and is read by executing RDSR

instruction. If this bit is “1” then the EEPROM is write enabled. If

this bit is “0” then the EEPROM is write disabled.