External Memory Controller (EMC)
Symphony DSP56724/DSP56725 Multi-Core Audio Processors, Rev. 0
Freescale Semiconductor
21-89
21.5.1.3
Peripheral Hierarchy on External Memory Controller for Very High Bus
Speeds
To achieve the highest possible bus speeds on the EMC, it is recommended to reduce even further the
number of devices connected directly to the EMC. For those cases, probably only one bank of synchronous
SRAMs or SDRAMs should be used, and instead of using a separate latch and a separate bus transceiver,
a bus demultiplexor combining those two functions into one device should be used.
shows
an example of such a hierarchy. This section is only a guideline, and the board designer must simulate the
electric characteristics of his scenario to determine the maximum operating frequency.
Figure 21-51. EMC Peripheral Hierarchy for Very High Bus Speeds
21.5.1.4
GPCM Timing
In the case where a system contains a memory hierarchy with high speed synchronous memories
(SDRAM, synchronous SRAM) and lower speed asynchronous memories (like flash EPROM,
peripherals, and others), then the GPCM controlled memories should be decoupled by buffers to reduce
capacitive loading on the bus. Those buffers have to be taken into account for the timing calculations.
Figure 21-52. GPCM Address Timing
To calculate address set-up timing for a slower peripheral/memory, several parameters have to be added:
•
Propagation delay for the address latch
•
Propagation delay for the buffer
•
Address set-up time for the actual peripheral
Typical values for the two propagation delays are in the order of 3–6 ns.
MA
Muxed Address and Data
Unmuxed Address
LA[2:0]
LAD[23:0]
LALE
LBCTL
EMC
Latch
A/D
LE
DIR
Q
B
Slower memories
and peripherals
Buffered Data
A
DQ
SDRAM
A
DQ
A
Device
input
A
pin
Buffer
Latch
LAD[23:0]
LALE
LBCTL
EMC
Slower memories
and peripherals
Unmuxed
Address
Buffered
Address
Muxed
Address and
Data