 2003 Microchip Technology Inc.

DS21160F-page 11

24LC21A

5.0

ACKNOWLEDGE POLLING

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

command has been issued from the master, the device

initiates the internally timed write cycle. ACK polling

can be initiated immediately. This involves the master

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 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 flow

diagram.

FIGURE 5-1:

ACKNOWLEDGE

POLLING FLOW

6.0

WRITE PROTECTION

When using the 24LC21A in the Bidirectional mode, the

VCLK pin can be used as a write-protect control pin.

Setting VCLK high allows normal write operations,

while setting VCLK low prevents writing to any location

allow the 24LC21A to operate as a serial ROM,

although this configuration would prevent using the

device in the Transmit-only mode.

Did Device

Acknowledge

(ACK = 0)?

Send

Write Command

Send Stop

Condition to

Initiate Write Cycle

Send Start

Send Control Byte

with R/W = 0

Next

Operation

No

Yes