NXP Semiconductors LPC24XX UM10237 - Manual (Page 72)

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User manual Rev. 04 — 26 August 2009 72 of 792
NXP Semiconductors
UM10237
Chapter 5: LPC24XX External Memory Controller (EMC)
• If the buffers are enabled, an AHB write operation writes into the Least Recently Used
(LRU) buffer, if empty.
If the LRU buffer is not empty, the contents of the buffer are flushed to memory to
make space for the AHB write data.
• If a buffer contains write data it is marked as dirty, and its contents are written to
memory before the buffer can be reallocated.
The write buffers are flushed whenever:
• The memory controller state machine is not busy performing accesses to external
memory.
The memory controller state machine is not busy performing accesses to external
memory, and an AHB interface is writing to a different buffer.
Note: For dynamic memory, the smallest buffer flush is a quadword of data. For static
memory, the smallest buffer flush is a byte of data.
5.4.2 Read buffers
Read buffers are used to:
• Buffer read requests from memory. Future read requests that hit the buffer read the
data from the buffer rather than memory, reducing transaction latency.
Convert all read transactions into quadword bursts on the external memory interface.
This enhances transfer efficiency for dynamic memory.
• Reduce external memory traffic. This improves memory bandwidth and reduces
power consumption.
Read buffer operation:
• If the buffers are enabled and the read data is contained in one of the buffers, the read
data is provided directly from the buffer.
• If the read data is not contained in a buffer, the LRU buffer is selected. If the buffer is
dirty (contains write data), the write data is flushed to memory. When an empty buffer
is available the read command is posted to the memory.
A buffer filled by performing a read from memory is marked as not-dirty (not containing
write data) and its contents are not flushed back to the memory controller unless a
subsequent AHB transfer performs a write that hits the buffer.
5.5 Memory controller state machine
The memory controller state machine comprises a static memory controller and a dynamic
memory controller.
6. Low-power operation
In many systems, the contents of the memory system have to be maintained during
low-power sleep modes. The EMC provides a mechanism to place the dynamic memories
into self-refresh mode.