Specifications

7.3. HARDWARE DESIGN

Figure 7.2: I

C master to slave multi-byte data exchange

Diagram based on illustration from [19]

An exchange from slave to master is similar, as illustrated in Figure 7.3.

The I

C speciﬁcation does not permit a slave to initiate a transfer

, and

thus the master must request data from the slave device. The only diﬀerence

between this scenario and the previous one, is that the master now asserts

the SDA line during the LSb of the address byte, thereby requesting data

from (rather than writing data to) the slave. The master will release the

SCL line at this stage, but the slave will hold it low while it prepares

data for transmission. When released, the master clocks all 8 bits and

then acknowledges receipt by pulling the SDL line low for a ninth bit cycle.

Following this byte e xchange, another byte is transmitted and this continues

until the master has received all it requires. At this point, the master does

not send an acknowledge bit, but rather a NACK followed by a stop bit. The

slave’s I

C interface is then reset and it awaits another start bit followed by

its address.

Figure 7.3: I

C slave to master multi-byte data exchange

Diagram based on illustration from [19]

The proto col used in this project consists of a single byte request from

unless it is a multi-master bus and the slave switches to master mode. But this

conﬁguration is outside the scope of this document as it is not used in this project