PIC12C5XX
device as part of a write operation. 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. It will then issue an
acknowledge and transmits the eight bit data word. The
master will not acknowledge the transfer but does gen-
erate a stop condition and the device discontinues
transmission (Figure 7-9). After this command, the
internal address counter will point to the address loca-
tion following the one that was just read.
7.5
READ OPERATIONS
Read operations are initiated in the same way as write
operations with the exception that the R/W bit of the
slave address is set to one. There are three basic types
of read operations: current address read, random read,
and sequential read.
7.5.1
CURRENT ADDRESS READ
It contains an address counter that maintains the
address of the last word accessed, internally incre-
mented by one. Therefore, if the previous read access
was to address n, the next current address read opera-
tion would access data from address n + 1. Upon
receipt of the slave address with the R/W bit set to one,
the device issues an acknowledge and transmits the
eight bit data word. The master will not acknowledge
the transfer but does generate a stop condition and the
device discontinues transmission (Figure 7-8).
7.5.3
SEQUENTIAL READ
Sequential reads are initiated in the same way as a ran-
dom read except that after the device transmits the first
data byte, the master issues an acknowledge as
opposed to a stop condition in a random read. This
directs the device to transmit the next sequentially
addressed 8-bit word (Figure 7-10).
7.5.2
RANDOM READ
To provide sequential reads, it contains an internal
address pointer which is incremented by one at the
completion of each read operation. This address
pointer allows the entire memory contents to be serially
read during one operation.
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
FIGURE 7-8: CURRENT ADDRESS READ
S
T
S
BUS ACTIVITY
A
CONTROL
BYTE
T
MASTER
R
O
P
T
SDA LINE
S
1 0 1 0 X X X 1
P
A
C
K
N
O
BUS ACTIVITY
DATA
A
C
K
X = Don’t Care Bit
FIGURE 7-9: RANDOM READ
S
T
A
R
T
S
T
A
R
T
S
T
O
P
BUS ACTIVITY
MASTER
CONTROL
BYTE
WORD
ADDRESS (n)
CONTROL
BYTE
X X X X
P
S
1 0 1 0 X X X 0
S
1 0 1 0 X X X 1
SDA LINE
A
C
K
A
C
K
A
C
K
N
O
DATA (n)
BUS ACTIVITY
A
C
K
X = Don’t Care Bit
FIGURE 7-10: SEQUENTIAL READ
S
T
O
P
BUS ACTIVITY
MASTER
CONTROL
BYTE
DATA n
DATA n + 1
DATA n + 2
DATA n + X
SDA LINE
P
A
C
K
A
C
K
A
C
K
A
C
K
N
O
BUS ACTIVITY
A
C
K
DS40139E-page 34
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
