Data Transfer Cycles; Read Cycle - Motorola MC68340 User Manual

3.3 DATA TRANSFER CYCLES

The transfer of data between the MC68340 and other devices involves the following
signals:
• Address Bus A31–A0
• Data Bus D15–D0
• Control Signals
The address bus and data bus are parallel, nonmultiplexed buses. The bus master moves
data on the bus by issuing control signals, and the bus uses a handshake protocol to
ensure correct movement of the data. In all bus cycles, the bus master is responsible for
de-skewing all signals it issues at both the start and end of the cycle. In addition, the bus
master is responsible for de-skewing the acknowledge and data signals from the slave
devices. The following paragraphs define read, write, and read-modify-write cycle
operations. Each bus cycle is defined as a succession of states that apply to the bus
operation. These states are different from the MC68340 states described for the CPU32.
The clock cycles used in the descriptions and timing diagrams of data transfer cycles are
independent of the clock frequency. Bus operations are described in terms of external bus
states.

3.3.1 Read Cycle

During a read cycle, the MC68340 receives data from a memory or peripheral device. If
the instruction specifies a long-word or word operation, the MC68340 attempts to read two
bytes at once. For a byte operation, the MC68340 reads one byte. The section of the data
bus from which each byte is read depends on the operand size, address signal A0, and
the port size. Refer to 3.2.1 Dynamic Bus Sizing and 3.2.2 Misaligned Operands for
more information. Figure 3-7 is a flowchart of a word read cycle.
ADDRESS DEVICE
1. SET R/W TO READ
2. DRIVE ADDRESS ON A31–A0
3. DRIVE FUNCTION CODE ON FC3–FC0
4. DRIVE SIZE PINS FOR OPERAND SIZE
5. ASSERT AS AND DS
1. LATCH DATA
2. NEGATE AS AND DS
START NEXT CYCLE
3-16
Freescale Semiconductor, Inc.
BUS MASTER
ACQUIRE DATA
Figure 3-7. Word Read Cycle Flowchart
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SLAVE
PRESENT DATA
1. DECODE ADDRESS
2. PLACE DATA ON D15–D0
3. DRIVE DSACKx SIGNALS
TERMINATE CYCLE
1. REMOVE DATA FROM D15–D0
2. NEGATE DSACKx
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