MCP79401 datasheet(21/42 Pages) MICROCHIP

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 2011 Microchip Technology Inc.

DS25009C-page 21

MCP7940X

FIGURE 6-3:

ID BYTE WRITE

6.2.2

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. The SRAM array can be read in

the same way as the ID using the control byte for the

SRAM ‘1101111’ with a valid address.

6.2.2.1

Current Address Read

The MCP7940X 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.

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

the MCP7940X 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

MCP7940X discontinues transmission (Figure 6-4).

FIGURE 6-4:

CURRENT ADDRESS

READ (ID SHOWN)

6.2.2.2

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

MCP7940X 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 MCP7940X 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 MCP7940X to discontinue

transmission (Figure 6-5). After a random read

command, the internal address counter will point to the

address location following the one that was just read.

6.2.2.3

Sequential Read

Sequential reads are initiated in the same way as a

random read except that after the MCP7940X 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 MCP7940X to

transmit the next sequentially addressed 8-bit word

(Figure 6-6). 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 MCP7940X 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.

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

CONTROL

BYTE

ADDRESS

BYTE

DATA

S 101 0

1 11

1 11 1 0 •• •

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

CONTROL

BYTE

DATA

BYTE

11 1

1 1 11 0

• • •

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