PIC12C67X datasheet(36/129 Pages) MICROCHIP

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PIC12C67X

DS30561B-page 36

6.3

Write Operations

6.3.1

BYTE WRITE

Following the start signal from the processor, the

device code (4 bits), the don’t care bits (3 bits), and the

R/W bit (which is a logic low) are placed onto the bus

by the processor. This indicates to the addressed

EEPROM that a byte with a word address will follow

after it has generated an acknowledge bit during the

ninth clock cycle. Therefore, the next byte transmitted

by the processor is the word address and will be written

into the address pointer. Only the lower four address

bits are used by the device, and the upper four bits are

don’t cares. If the address byte is acknowledged, the

processor will then transmit the data word to be written

into the addressed memory location. The memory

acknowledges again and the processor generates a

stop condition. This initiates the internal write cycle,

and during this time will not generate acknowledge sig-

nals. After a byte write command, the internal address

counter will not be incremented and will point to the

same address location that was just written. If a stop bit

sequence is transmitted to the device at any point in the

write command sequence before the entire sequence

is complete, then the command will abort and no data

will be written. If more than 8 data bits are transmitted

before the stop bit sequence is sent, then the device will

clear the previously loaded byte and begin loading the

data buffer again. If more than one data byte is trans-

mitted to the device and a stop bit is sent before a full

eight data bits have been transmitted, then the write

command will abort and no data will be written. The

EEPROM memory employs a VCC threshold detector

circuit, which disables the internal erase/write logic if

the VCC is below minimum VDD. Byte write operations

must be preceded and immediately followed by a bus

not busy bus cycle where both SDA and SCL are held

high. (See Figure 6-7 for Byte Write operation.)

6.4

Acknowledge Polling

Since the EEPROM will not acknowledge during a write

cycle, this can be used to determine when the cycle is

complete (this feature can be used to maximize bus

throughput). Once the stop condition for a write com-

mand has been issued from the processor, the device

initiates the internally timed write cycle. ACK polling

can be initiated immediately. This involves the proces-

sor sending a start condition followed by the control

byte for a write command (R/W = 0). If the device is still

busy with the write cycle, then no ACK will be returned.

If no ACK is returned, then the start bit and control byte

must be re-sent. If the cycle is complete, then the

device will return the ACK and the processor can then

proceed with the next read or write command. (See

Figure 6-6 for flow diagram.)

FIGURE 6-6:

ACKNOWLEDGE POLLING

FLOW

FIGURE 6-7:

BYTE WRITE

Send

Write Command

Send Stop

Condition to

Initiate Write Cycle

Send Start

Send Control Byte

with R/W = 0

Did EEPROM

Acknowledge

(ACK = 0)?

Operation

YES

SDA LINE

CONTROL

BYTE

WORD

ADDRESS

DATA

X = Don’t Care Bit

ACTIVITY

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