MCP79411 datasheet(41/60 Pages) MICROCHIP

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DS20002266E-page 41

MCP79410/MCP79411/MCP79412

10.2.5

READ OPERATION

Read operations are initiated in the same way as write

operations with the exception that the R/W bit of the

control byte is set to one. There are three basic types

of read operations: current address read, random read,

and sequential read.

10.2.5.1

Current Address Read

The MCP7941X contains an address counter that

maintains the address of the last word accessed,

internally incremented by one. Therefore, if the

previous read access was to address n (n is any legal

address), the next current address read operation

would access data from address n + 1. In the case of a

page write, if the last byte written is the last byte of a

page, the next address location would be the first byte

of the same page written.

Upon receipt of the control byte with R/W bit set to one,

the MCP7941X issues an Acknowledge and transmits

the 8-bit data word. The master will not acknowledge

the transfer but does generate a Stop condition and the

MCP7941X discontinues transmission (Figure 10-1).

FIGURE 10-1: CURRENT ADDRESS READ

(EEPROM SHOWN)

10.2.5.2

Random Read

Random read operations allow the master to access

any memory location in a random manner. To perform

this type of read operation, first the word address must

be set. This is done by sending the word address to the

MCP7941X as part of a write operation (R/W bit set to

‘0’). After the word address is sent, the master

generates a Start condition following the Acknowledge.

This terminates the write operation, but not before the

internal Address Pointer is set. Then, the master issues

the control byte again but with the R/W bit set to a one.

The MCP7941X will then issue an Acknowledge and

transmit the 8-bit data word. The master will not

acknowledge the transfer but it does generate a Stop

condition which causes the MCP7941X to discontinue

transmission (Figure 10-2). After a random Read

command, the internal address counter will point to the

address location following the one that was just read.

10.2.5.3

Sequential Read

Sequential reads are initiated in the same way as a

random read except that after the MCP7941X transmits

the first data byte, the master issues an Acknowledge

as opposed to the Stop condition used in a random

read. This Acknowledge directs the MCP7941X to

transmit the next sequentially addressed 8-bit word

(Figure 10-3). Following the final byte transmitted to the

master, the master will NOT generate an Acknowledge

but will generate a Stop condition. To provide

sequential reads, the MCP7941X contains an internal

Address Pointer which is incremented by one at the

completion of each operation. This Address Pointer

allows the entire memory contents to be serially read

during one operation. The internal Address Pointer will

automatically roll over to the start of the Block.

FIGURE 10-2: RANDOM READ (EEPROM SHOWN)

FIGURE 10-3: SEQUENTIAL READ (EEPROM SHOWN)

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

CONTROL

BYTE

DATA

BYTE

11 1

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

CONTROL

BYTE

ADDRESS

BYTE

CONTROL

BYTE

DATA

BYTE

S 1 010

111

S 10 10

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

CONTROL

BYTE

DATA n

DATA n + 1

DATA n + 2

DATA n + X

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