6

AT90SP0801

1495AS–01/02

Read Commands

Block read commands are slightly different than writes and are encoded as follows:

The read command is only one byte long, and the chip (not the host) sends back the

count information. The count value will always be the smaller of MAXBLK_R or the

(remaining) number of bytes in the register that have not been read yet.

When there are a large number of bytes in the buffer, multiple read commands must be

executed to read all the bytes out of the chip. Using the slave address of 0x53 will cause

the chip to start reading at the beginning of the buffer. Using the slave address of 0x51

will cause the chip to continue reading information that is subsequent to the information

last read by the chip from the buffer. After a load or crypto operation, the first command

may also be a 0x51, which will have the same effect as 0x53.

Block Reads are formatted as follows:

After the last byte has been read from the register, the read pointer is reset back to the

beginning of the register, and the system may continue to read from the beginning of the

buffer again, if desired. There is no indication from the chip as to when the read pointer

has been reset (other than as may be inferred from the values in the count field).

To be compatible with the SMBus specification, the read command may optionally be

preceded by the first two bytes of either of the “ignored write” commands, which are then

aborted with a new start bit for the read. The two bytes of the write command are com-

pletely ignored by the chip in this case, and a different encoding for the second byte

(01111111, or 0x7F) must be used. Execution of a block read sequence using a legal

write command code for the second byte (00, 0x40, 0x80 or 0xC) is undefined.

The protocol for this is shown below:

Slave Address

Command Code

Description

0 101 00 11

--------

Read buffer, first block

0 101 00 01

--------

Read, subsequent blk

17

1

1

8

1

...

S

Slave Address

Rd

A

Byte Count = N

A

8

181

...

81

1

Data Byte 1

A

Data Byte 2

A

Data Byte N

NP

17

1

1

8

1

1

7

1

1

...

S

Slave Address

Wr

A

0111 1111

AS

Slave Address

Rd

A

81

8

1

8

1

...

81

1

Byte Count = N

A

Data byte 1

A

Data byte 2

A

Data byte N

NP