X25020

4

Clock and Data Timing

Data input on the SI line is latched on the rising edge of

SCK. Data is output on the SO line by the falling edge of

SCK.

Read Sequence

CS is

first pulled LOW to select the device. The 8-bit READ

instruction is transmitted to the X25020, followed by the

8-bit address. After the READ opcode and address are

sent, the data stored in the memory at the selected

address is shifted out on the SO line. The data stored

in memory at the next address can be read sequentially

by continuing to provide clock pulses. The address is

automatically incremented to the next higher address

after each byte of data is shifted out. When the highest

address is reached ($FF) the address counter rolls

over to address $00 allowing the read cycle to be

continued indefinitely. The read operation is termi-

nated by taking

array operation sequence illustrated in Figure 1.

To read the status register

CS line is first pulled LOW

to select the device followed by the 8-bit RDSR instruc-

tion. After the read status register opcode is sent, the

contents of the status register are shifted out on the SO

line. Figure 2 illustrates the read status register se-

quence.

Write Sequence

Prior to any attempt to write data into the X25020 the

“write enable” latch must first be set by issuing the

WREN instruction (See Figure 3).

CS is first taken LOW,

then the WREN instruction is clocked into the X25020.

After all eight bits of the instruction are transmitted,

CS

must then be taken HIGH. If the user continues the write

operation without taking

CS HIGH after issuing the

WREN instruction, the write operation will be ignored.

issues the WRITE instruction, followed by the address

and then the data to be written. This is minimally a thirty-

two clock operation.

CS must go LOW and remain LOW

for the duration of the operation. The host may continue

to write up to 4 bytes of data to the X25020. The only

restriction is that the 4 bytes must reside on the same

page. If the address counter reaches the end of the page

and the clock continues, the counter will “roll over” to the

first address of the page and overwrite any data that may

have been written.

For the write operation (byte or page write) to be

completed,

CS can only be brought HIGH after bit 0 of

data byte N is clocked in. If it is brought HIGH at any other

time the write operation will not be completed. Refer to

Figures 4 and 5 below for a detailed illustration of the

write sequences and time frames in which

CS going

HIGH are valid.

To write to the status register, the WRSR instruction is

followed by the data to be written. Data bits 0, 1, 4, 5, 6

and 7 must be “0”. Figure 6 illustrates this sequence.

While the write is in progress following a status register

read to check the WIP bit. During this time the WIP bit will

be HIGH.

Hold Operation

The

operation. If a data transfer is to be interrupted

HOLD

can be pulled LOW to suspend the transfer until it can be

resumed. The only restriction is the SCK input must be

LOW when

HOLD is first pulled LOW and SCK must also

be LOW when

HOLD is released.