Service manual

CONFIDENTIAL – DO NOT COPY
Page 7-12
File No. SG-0198
2. Mode Register Set (MRS)
The mode register stores the data for controlling the various operating modes of DDR SDRAM. It
programs CAS latency, addressing mode, burst length, test mode, DLL reset and various vendor
specific options to make DDR SDRAM useful for a variety of different applications. The default value
of the mode register is not defined, therefore the mode register must be written after EMRS setting
for proper DDR SDRAM operation.
The mode register is written by asserting low on CS, RAS, CAS, WE and BA0 (The DDR SDRAM
should be in all bank precharge with CKE already high prior to writing into the mode register). The
state of address pins A0 ~ A11 in the same cycle as CS, RAS, CAS, WE and BA0 low is written in
the mode register. Two clock cycles are required to meet tMRD spec. The mode register contents
can be changed using the same command and clock cycle requirements during operation as long as
all banks are in the idle state. The mode register is divided into various fields depending on
functionality. The burst length uses A0 ~ A2, addressing mode uses A3, CAS latency (read latency
from column address) uses A4 ~ A6. A7 is a ProMOS specific test mode during production test. A8 is
used for DLL reset. A7 must be set to low for normal MRS operation. Refer to the table for specific
codes for various burst length, addressing modes and CAS latencies.
1. MRS can be issued only at all banks precharge state.
2. Minimum tRP is required to issue MRS command.