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NXP Semiconductors
PNX15xx/952x Series
Volume 1 of 1
Chapter 9: DDR Controller
PNX15XX_PNX952X_SER_N_4
NXP B.V. 2007. All rights reserved.
Product data sheet
Rev. 4.0 — 03 December 2007
9-332
In general, setting the cpu_limit too low will block the CPU too frequently causing a
too high latency (execution time). Setting the cpu_limit too high can completely block
the soft real time DMA for a long time when the hard real time DMA and CPU
bandwidth are peaking. But perhaps the long latency that causes the soft real time
may not be a problem.
3.6 The DDR Controller and the DDR Memory Devices
The DDR SDRAM Controller is compatible with most of the DDR SDRAM vendors.
This is achieved when the correct timing parameters are programmed in the MMIO
registers holdings the timing parameters has presented in the two following sections.
4.
Timing Diagrams and Tables
This section shows how programmable timing parameters direct the operation of the
DDR SDRAM Controller. It is not the intention of this section to give a complete
overview of all DDR interface signaling. Only the main ones are described.
Table 6 presents the values that are used for the different timing parameters in the
timing diagrams.
Throughout all timing diagrams a DDR burst size of eight data elements is used.
In the timing diagrams, symbols are used to indicate the DDR commands that are
issued by the DDR controller. An overview of these commands and their symbol
Table 6: DDR Timing Parameters
Parameter
Symbol
Value (Clock
Cycles)
CAS latency
tCAS
2.5
Minimum time between two active commands to different banks
tRRD
3
Minimum time between two active commands to same bank
tRC
8
Minimum time between auto refresh and active command
tRFC
8
Minimum time after last data write and precharge to same bank
tWR
1
Minimum time between active and precharge command
tRAS
8
Minimum time between precharge and active command
tRP
4
Minimum time between active and read command
tRCD_RD
4
Minimum time between active and write command
tRCD_WR
2
Table 7: DDR Commands
DDR Commands
Symbol
Any DDR command
Any
Activate command
Act
Precharge command
Pre
Read command
Read
Write command
Write
Auto refresh command
A. rf.