External Memory Controller (EMC)
Symphony DSP56724/DSP56725 Multi-Core Audio Processors, Rev. 0
Freescale Semiconductor
21-41
21.4.1.2
External Address Latch Enable Signal (LALE)
The external memory bus uses a multiplexed address and data bus. Therefore, the EMC must distinguish
between address and data phases, which take place on the same bus (LAD[23:0] pins). The LALE signal,
when asserted, signifies an address phase during which the EMC drives the memory address on the
LAD[23:0] pins. An external address latch uses the LALE signal to capture the address and provide it to
the address pins of the memory or peripheral device. When LALE is negated, LAD[23:0] then serves as
the (bidirectional) data bus for the access. Any address phase initiates the assertion of LALE, which has a
programmable duration of 1–4 bus clock cycles.
The frequency of LALE assertion varies across the three memory controllers. In the case of GPCM, every
assertion of LCSx is considered an independent access, and accordingly, LALE will assert prior to each
such access. For example, GPCM would assert LALE and LCSx 8 times to satisfy an 8-words cache line
transfer. The SDRAM controller asserts LALE only to initiate a burst transfer with a starting address,
therefore no more than one assertion of LALE may be required for SDRAM to transfer a 8-word cache
line.
In the case of UPM, the frequency of LALE assertion depends on how the UPM RAM is programmed.
UPM single accesses typically assert LALE once, upon commencement, but it is possible to program UPM
to assert LALE several times, and to change the values of LA[2:0] with and without LALE being involved.
In general, when using the GPCM and SDRAM controllers it is not necessary to use LA[2:0] if a
sufficiently wide latch is used to capture the entire address during LALE phases. UPM may require
LA[2:0] if EMC is generating its own burst address sequence.
21.4.1.3
Data Transfer Acknowledge (TA)
The three memory controllers in the EMC generate an internal transfer acknowledge signal (TA), to allow
data on LAD[23:0] to be either sampled (for reads) or changed (on writes). The data sampling/data change
always occurs at the end of the bus cycle in which TA is asserted internally by the EMC. GPCM and
SDRAM controllers automatically generate TA according to the timing parameters programmed for them
in option and mode registers; UPM generates TA only when a UPM pattern has the UTA RAM word bit
set. An illustration of LALE, TA (internal), and LCSx is shown in
. Note that TA and LALE
are never asserted together, and that for the duration of LALE, LCSx (or any other control signal) remains
negated or frozen.
Figure 21-3. Basic EMC Bus Cycle with LALE, TA, and LCSx
LAD
LALE
LCSx
TA
LCLK
Address
Data
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