Datasheet

MCP7940N
DS20005010F-page 36  2010-2014 Microchip Technology Inc.
6.1.3 SRAM/RTCC REGISTER CURRENT
ADDRESS READ
The MCP7940N contains an address counter that
maintains the address of the last byte accessed, inter-
nally 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 ‘1’,
the MCP7940N 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
MCP7940N discontinues transmission (Figure 6-3).
FIGURE 6-3: SRAM/RTCC CURRENT
ADDRESS READ
6.1.4 SRAM/RTCC REGISTER 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 address must be
set. This is done by sending the address to the
MCP7940N as part of a write operation (R/W bit set to
‘0’). After the address is sent, the master generates a
Start condition following the Acknowledge. This termi-
nates 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 ‘1’. The
MCP7940N will then issue an Acknowledge and trans-
mit the 8-bit data word. The master will not acknowl-
edge the transfer but it does generate a Stop condition
which causes the MCP7940N to discontinue transmis-
sion (Figure 6-4). After a random Read command, the
internal address counter will point to the address loca-
tion following the one that was just read.
6.1.5 SRAM/RTCC REGISTER
SEQUENTIAL READ
Sequential reads are initiated in the same way as a
random read except that after the MCP7940N trans-
mits the first data byte, the master issues an Acknowl-
edge as opposed to the Stop condition used in a
random read. This Acknowledge directs the
MCP7940N to transmit the next sequentially
addressed 8-bit word (Figure 6-5). Following the final
byte transmitted to the master, the master will NOT
generate an Acknowledge but will generate a Stop con-
dition. To provide sequential reads, the MCP7940N
contains an internal Address Pointer which is incre-
mented by one at the completion of each operation.
This Address Pointer allows the entire memory block to
be serially read during one operation.
Because the RTCC registers and SRAM are separate
blocks, reading past the end of each block will cause
the Address Pointer to roll over to the beginning of the
same block. Specifically, the Address Pointer will roll
over from 0x1F to 0x00, and from 0x5F to 0x20.
FIGURE 6-4: SRAM/RTCC RANDOM READ
FIGURE 6-5: SRAM/RTCC SEQUENTIAL READ
BUS ACTIVITY
MASTER
SDA LINE
BUS ACTIVITY
PS
S
T
O
P
CONTROL
BYTE
S
T
A
R
T
DATA
A
C
K
N
O
A
C
K
1011 1
BYTE
111
BUS ACTIVITY
MASTER
SDA LINE
BUS ACTIVITY
A
C
K
N
O
A
C
K
A
C
K
A
C
K
S
T
O
P
S
T
A
R
T
CONTROL
BYTE
ADDRESS
BYTE
CONTROL
BYTE
DATA
BYTE
S
T
A
R
T
S
1101 0111
S 1101 1 P111
BUS ACTIVITY
MASTER
SDA LINE
BUS ACTIVITY
CONTROL
BYTE
DATA n DATA n + 1 DATA n + 2 DATA n + X
N
O
A
C
K
A
C
K
A
C
K
A
C
K
A
C
K
S
T
O
P
P