PIC12C5XX
7.3
WRITE OPERATIONS
7.4
ACKNOWLEDGE POLLING
7.3.1
BYTE WRITE
Since the device 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 master, the device ini-
tiates the internally timed write cycle. ACK polling can
be initiated immediately. This involves the master send-
ing 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 master can then proceed with
the next read or write command. See Figure 7-6 for
flow diagram.
Following the start signal from the master, 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
master transmitter. This indicates to the addressed
slave receiver 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 master 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. The address byte is acknowledgeable and
the master device will then transmit the data word to be
written into the addressed memory location. The mem-
ory acknowledges again and the master generates a
stop condition. This initiates the internal write cycle,
and during this time will not generate acknowledge sig-
nals (Figure 7-7). After a byte write command, the inter-
nal address counter will not be incremented and will
point to the same address location that was just written.
If a stop bit 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 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
transmitted 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.
FIGURE 7-6: ACKNOWLEDGE POLLING
FLOW
Send
Write Command
Send Stop
Condition to
Initiate Write Cycle
Send Start
Send Control Byte
with R/W = 0
Byte write operations must be preceded and immedi-
ately followed by a bus not busy bus cycle where both
SDA and SCL are held high.
Did Device
NO
Acknowledge
(ACK = 0)?
YES
Next
Operation
FIGURE 7-7: BYTE WRITE
S
T
A
R
T
S
T
O
P
BUS ACTIVITY
MASTER
CONTROL
BYTE
WORD
ADDRESS
DATA
SDA LINE
P
S
1
0
1
0
X
X
X
0
X
X
X
X
A
C
K
A
C
K
A
C
K
BUS ACTIVITY
X = Don’t Care Bit
1999 Microchip Technology Inc.
DS40139E-page 33
MICROCHIP [ MICROCHIP ]
