Address Phase After Previous Read; Read Data Phase After Address Phase; Read-Modify-Write Cycle For Parity Protected Memory Banks; Upm Cycles With Additional Address Phases - Freescale Semiconductor MPC8313E Family Reference Manual

Enhanced Local Bus Controller
• Continued burst after the first beat
• Write data phase after address phase
• Address phase after previous write
10.5.2.1

Address Phase after Previous Read

During a read cycle, the memory/peripheral drives the bus and the bus transceiver drives LAD. After the
data has been sampled, the output drivers of the external device must be disabled. This can take some time;
for slow devices the EHTR feature of the GPCM or the programmability of the UPM should be used to
guarantee that those devices have stopped driving the bus when the eLBC memory controller ends the bus
cycle.
In this case, after the previous cycle ends, LBCTL goes high and changes the direction of the bus
transceiver. The eLBC then inserts a bus turnaround cycle to avoid contention. The external device has
now already placed its data signals in high impedance and no bus contention will occur.
10.5.2.2

Read Data Phase after Address Phase

During the address phase, LAD actively drives the address and LBCTL is high, driving the bus
transceivers in the same direction as during a write. After the end of the address phase, LBCTL goes low
and changes the direction of the bus transceiver. The eLBC places the LAD signals in high impedance after
its t (LB). The LBCTL will have its new state after t
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the transceiver starts to drive those signals after its t
ensure, that [t (LB) + t
en
10.5.2.3

Read-Modify-Write Cycle for Parity Protected Memory Banks

Principally, a read-modify-write cycle is a read cycle immediately followed by a write cycle. Because the
write cycle will have a new address phase in any case, this basically is the same case as an address phase
after a previous read.
10.5.2.4

UPM Cycles with Additional Address Phases

The flexibility of the UPM allows the user to insert additional address phases during read cycles by
changing the AMX field, therefore turning around the bus during one pattern. The eLBC automatically
inserts a single bus turnaround cycle if the bus (LAD) was previously high impedance for any reason, such
as a read, before LALE is driven and LAD is driven with the new address. The turnaround cycle is not
inserted on a write, because the bus was already driven to begin with.
However, bus contention could potentially still occur on the far side of a bus transceiver. It is the
responsibility of the designer of the UPM pattern to guarantee that enough idle cycles are inserted in the
UPM pattern to avoid this.
10.5.3

Interface to Different Port-Size Devices

The eLBC supports 8- and 16-bit data port sizes. However, the bus requires that the portion of the data bus
used for a transfer to or from a particular port size be fixed. A 16-bit port must reside on LAD[0–15], and
MPC8313E PowerQUICC II Pro Integrated Processor Family Reference Manual, Rev. 3
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(transceiver)] is larger than t
en
(LB) and, because this is an asynchronous input,
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(transceiver) time. The system designer has to
en
(LB) to avoid bus contention.
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