[U-Boot-Users] Porting U-Boot - SDRAM mode register set command - question

Peter Asemann peter.asemann at web.de
Tue Dec 14 18:19:34 CET 2004

I have some problems to understand how to initialize SDRAM... probably 
this is a little off topic... please forgive me.

I need to program the mode register of the SDRAM in order to get the 
SDRAM working.

So I'm going to write some opcode in the MAR of my MPC875, issue a "run" 
command to the MCR and have a Ram-Word executed which sets the 
CS/RAS/CAS/WE lines correctly so the SDRAM will know I'm trying to issue 
a mode register set command.

The content of the MAR will be put on the address lines, as I'll set AMX 
of the ram-word to 11.
The SDRAM won't see bits 30 and 31, as I have a 32-bit bus and these 
lines are not connected, so I'll take care of that when assembling the 
contents of MAR.
The SDRAM also won't see bit 19, as it's not connected, too - I'll have 
to make GPL_A0 be in the right state in the ram-word instead (which I 
find not very elegant, but I don't see a way to have the MPC875 assign 
the state of address line A19 to GPL_A0).

Okay... that's not such a big problem in theory (correct me if necessary).

The problem is that my SDRAM data sheet says it takes/samples the opcode 
for the mode register set command from its input lines BA0/1, AP/A10, 
A11-12 and A0-9 (I count 15 bit).

To my sorrow this data sheet doesn't mention any opcode. I needed to 
search half the internet to find information on that issue.

The Internet ressource I found says the mode register is only 12 (not 
15) bits wide and only the least significant 7 bits are in use.

So I'm confused... where can I find reliable information on SDRAM mode 
register opcodes, and how many bits is an SDRAM mode register wide?

Peter Asemann

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