5

1937J–DFLSH–9/05

AT45DB021B

A low-to-high transition on the CS pin will terminate the read operation and tri-state the SO pin.

The maximum SCK frequency allowable for the Continuous Array Read is defined by the f

CAR

specification. The Continuous Array Read bypasses both data buffers and leaves the contents

of the buffers unchanged.

5.1.2

Main Memory Page Read

A Main Memory Page Read allows the user to read data directly from any one of the 1024 pages

in the main memory, bypassing both of the data buffers and leaving the contents of the buffers

unchanged. To start a page read, an opcode of 52H or D2H must be clocked into the device fol-

lowed by 24 address bits and 32 don’t care bits. The first five bits of the 24-bit address sequence

are reserved bits, the next 10 address bits (PA9 -PA0) specify the page address, and the next

nine address bits (BA8 - BA0) specify the starting byte address within the page. The 32 don’t

care bits which follow the 24 address bits are sent to initialize the read operation. Following the

32 don’t care bits, additional pulses on SCK result in serial data being output on the SO (serial

output) pin. The CS pin must remain low during the loading of the opcode, the address bits, the

don’t care bits and the reading of data. When the end of a page in main memory is reached dur-

ing a Main Memory Page Read, the device will continue reading at the beginning of the same

page. A low-to-high transition on the CS pin will terminate the read operation and tri-state the

SO pin.

5.1.3

Buffer Read

Data can be read from either one of the two buffers, using different opcodes to specify which

buffer to read from. An opcode of 54H or D4H is used to read data from buffer 1, and an opcode

of 56H or D6H is used to read data from buffer 2. To perform a Buffer Read, the eight bits of the

opcode must be followed by 15 don’t care bits, nine address bits, and eight don’t care bits. Since

the buffer size is 264-bytes, nine address bits (BFA8 -BFA0) are required to specify the first byte

of data to be read from the buffer. The CS pin must remain low during the loading of the opcode,

the address bits, the don’t care bits and the reading of data. When the end of a buffer is reached,

the device will continue reading back at the beginning of the buffer. A low-to-high transition on

the CS pin will terminate the read operation and tri-state the SO pin.

5.1.4

Status Register Read

The status register can be used to determine the device’s ready/busy status, the result of a Main

Memory Page to Buffer Compare operation, or the device density. To read the status register,

an opcode of 57H or D7H must be loaded into the device. After the last bit of the opcode is

shifted in, the eight bits of the status register, starting with the MSB (bit 7), will be shifted out on

the SO pin during the next eight clock cycles. The five most-significant bits of the status register

will contain device information, while the remaining three least-significant bits are reserved for

future use and will have undefined values. After bit 0 of the status register has been shifted out,

the sequence will repeat itself (as long as CS remains low and SCK is being toggled) starting

again with bit 7. The data in the status register is constantly updated, so each repeating

sequence will output new data.

Ready/Busy status is indicated using bit 7 of the status register. If bit 7 is a 1, then the device is

not busy and is ready to accept the next command. If bit 7 is a 0, then the device is in a busy

state. The user can continuously poll bit 7 of the status register by stopping SCK at a low level

Table 5-1.

Status Register Format

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

RDY/BUSY

COMP

01

01

X

X