24LC41
5.0
ACKNOWLEDGE POLLING
6.0
6.1
WRITE PROTECTION
DDC Monitor Port
Acknowledge polling can be done for both the DDC
Monitor Port (when in bi-directional Mode) and the
Microcontroller Access Port.
Since the port 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 but
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 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 5-1 for the 铿俹w diagram.
When using the DDC Monitor Port in the bi-directional
Mode, the VCLK/DWP pin operates as the write protect
control pin. Setting VCLK/DWP high allows normal
write operations, while setting VCLK/DWP low prevents
writing to any location in the array. Connecting the
VCLK/DWP pin to V
SS
would allow the DDC Monitor
Port to operate as a serial ROM, although this con铿乬u-
ration would prevent using the device in the Transmit-
Only Mode.
6.2
Microcontroller Access Port
FIGURE 5-1:
ACKNOWLEDGE POLLING
FLOW
Send
Write Command
The Microcontroller Access Port can be used as a
serial ROM when the MWP pin is connected to V
CC
.
Programming will be inhibited and the entire memory
associated with the Microcontroller Access Port will be
write-protected.
7.0
READ OPERATION
Send Stop
Condition to
Initiate Write Cycle
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. These operations are identical for
both the DDC Monitor Port (in bi-directional Mode) and
the Microcontroller Access Port and are completely
independent of one another.
Send Start
7.1
Current Address Read
Send Control Byte
with R/W = 0
Did Device
Acknowledge
(ACK = 0)?
YES
Next
Operation
NO
The port contains an address counter that maintains
the address of the last word accessed, internally incre-
mented by one. Therefore, if the previous access (either
a read or write operation) was to address n, the next
current address read operation would access data from
address n + 1. Upon receipt of the slave address with
R/W bit set to one, the port issues an acknowledge and
transmits the 8-bit data word. The master will not
acknowledge the transfer but does generate a stop con-
dition and the port discontinues transmission (Figure 7-
1).
7.2
Random Read
Random read operations allow the master to access
any memory location in a random manner. To perform
this type of read operation, 铿乺st the word address must
be set. This is done by sending the word address to the
port 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. The
master then issues the control byte again, but with
the R/W bit set to a one. The port then issues an
acknowledge and transmits the 8-bit data word. The
master will not acknowledge the transfer but does gen-
erate a stop condition and the port discontinues trans-
mission (Figure 7-2).
漏
1996 Microchip Technology Inc.
DS21140B-page 9
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