RAM Timings Explained

This is not MY original work, but I thought it might be helpful to share these with everyone:

Cas# Latency (tCL)
Ras# to Cas# Delay (tRTC)
Ras# Precharge (tRP)
Active to Precharge (tRas)
Row Cycle Time (tRC)
Refresh to Activate Delay / Refresh Cycle Time (tRFC)
Refresh Mode Select (RMS) / Refresh Period (tREF)
Command Rate / Command per Clock (1T/2T)
Performance Level / Read Delay (tRD)
Write to Precharge Delay / Write Recovery Time (tWR)
Write to Read command Delay / Write to Read Delay (tWTR)
Activate to Activate delay (tRRD)
Read to Precharge delay (tRTP)
Read to Write delay (tRTW)
Precharge to Precharge delay (tPTP)
Write-Read Command Spacing (tWR-RD)
Read-Write Command Spacing (tRD-WR)
Write-Write Command Spacing (tWR-WR)
Force Auto Precharge
Maximum Asynchronous Latency
Maximum Read Latency
Read/Write Queue Bypass
Queue Bypass Max
DRAM Idle timer

-Cas# Latency (tCL).
Number of clocks that elapses between the memory controller telling
the memory module to access a particular column in the current row,
and the data from that column being read from the module's output pins.

-RAS# to CAS# Delay (tRCD).
Controls the number of clocks inserted between a row activate command
and a read or write command to that row. Last Intel chipset (965 and P35)
allow to change RAS# to CAS# Read Delay and RAS# to CAS# Write Delay separately

-RAS# Precharge (tRP).
Controls the number of clocks that are inserted between a row precharge
command and an activate command to the same rank.

-Activate to Precharge delay (tRAS).
Number of clocks taken between a bank active command and issuing the
precharge command. Usually, tRAS=tCL + tRCD + 2.

-Row Cycle Time (tRC).
Determines the minimum number of clock cycles a memory row takes to
complete a full cycle, from row activation up to the precharging of
the active row. For optimal performance, use the lowest value you can,
according to the tRC = tRAS + tRP formula. For example:
if your memory module's tRAS is 7 clock cycles and its tRP is 4 clock cycles,
then the row cycle time or tRC should be 11 clock cycles.

-Refresh to Activate Delay / Refresh Cycle Time (tRFC).
Determines the number of clock measured from a Refresh command (REF)
until the first Activate command (ACT) to the same rank

-Refresh Mode Select (RMS) / Refresh Period (tREF).
Determines at what rate refreshes will be executed. Contrary to other timings,
higher value is better for performance.

-Command Rate / Command per Clock (1T/2T).
Delay between when a memory chip is selected and when the first active
command can be issued. The factors that determine whether a memory
subsystem can tolerate a 1T command rate are many, including the number
of memory banks, the number of DIMMs present, and the quality of the DIMMs.

-Performance Level / Read Delay (tRD).
tRD is the number of memory clocks from DRAM Chip Select# assert
to Host Data Ready# assertion on the FSB.
Hight influence on performance and stability.

-Write to Precharge Delay / Write Recovery Time (tWR).
-Write Recovery time is an internal dram timing, values are usually 3 to 10.
It specifies the amount of delay (in clock cycles) that must elapse after the
completion of a valid write operation, before an active bank can be precharged.
-Write to Precharge is a command delay, and is calculed as:
Write to Precharge = tCL - 1 +BL/2 + tWR.
BL(Burst Lenght) practically always 8.

-Write to Read command Delay / Write to Read Delay (tWTR).
-Write to Read delay is an internal dram timing, values are usually 2 to 8.
Specifie the number of clock between the last valid write operation and the next
read command to the same internal bank
-Write to Read command is a command delay, and is calculed as:
Write to Read = tCL - 1 +BL/2 + tWTR.
BL(Burst Lenght) practically always 8.

-Activate to Activate delay (tRRD).
Number of clocks between two row activate in different banks of the same rank.

-Read to Precharge delay (tRTP).
Number of clocks that are inserted between a read command to a row
pre-charge command to the same rank.

-Read to Write delay (tRTW).
Number of clocks that are inserted between a read command to a write
command to the same rank.

-Precharge to Precharge delay (tPTP).
Number of clocks that are inserted between two Precharge command in
different banks of the same rank.

-Write-Read Command Spacing (tWR-RD).
This field determines the number of turn-around clocks on the data bus needs
to be inserted between write command and a subsequent read command on Different Rank.

-Read-Write Command Spacing (tRD-WR).
This field determines the number of turn-around clocks on the data bus needs
to be inserted between read command and a subsequent write command on Different Rank.

-Write-Write Command Spacing (tWR-WR).
This field controls the turnaround time on the DQ bus for WR-WR sequence to
different ranks in one channel.

-Force Auto Precharge.
When enabled, force auto Precharging with every read or write command.
This may be preferred in situation where powers savings is favored over performance.

-Maximum Asynchronous Latency.
Specify the maximum round trip latency in the system from the processeur to
the DRAM devices and back.

-Maximum Read Latency.
Specify the maximum round trip latency in the system from the processeur to
the DRAM devices and back. This time is specified in NorthBridge clock and
includes the asynchronous and synchronous latencies.

-Read/Write Queue Bypass
Specify the number of times that the oldest operation in the DCI read/Write
queue may be bypassed .

-Queue Bypass Max
Specify the maximum of times that the oldest memory-access request in
the DRAM controller queue may be bypassed .

-DRAM Idle timer.
Determine the number of clocks the DRAM Controller will remain in the idle
state before it begins precharging all pages.

Original document is found here.

Hope this helps!