Hardware Reference
In-Depth Information
Signal floating
(a)
Signal floating
(b)
Figure 14.29
■
Floating signals: (a) single signal, (b) multiple signal
A bus transaction includes sending the address and receiving or sending the data. A
read
transaction (also called a
read bus cycle
) transfers data from memory to either the CPU or the
I/O device, and a
write
transaction (also called a
write bus cycle
) writes data to the memory. In
a read transaction, the address is first sent down the bus to the memory, together with the ap-
propriate control signals indicating a read. In Figure 14.30, this means pulling the read signal to
high. The memory responds by placing the data on the bus and driving the Ready signal to low.
The Ready signal (asserted low) indicates that the data on the data bus is valid.
In Figure 14.30, a read bus cycle takes one clock cycle to complete. For some microcon-
trollers, the Ready signal is used to extend the bus cycle to more than one clock cycle to
End of read
transaction
Clock
Address
Data
Read
Ready
Figure 14.30
■
A typical bus read transaction
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