RE: ADDR []and DATA[] connections for 8 & 16 bit memory

You are correct about the data bus - data[0:7] for the 8-bit (bit 7 is the LSB) and data[0:15] for a 16-bit port (bit 16 is the LSB).

For an 8-bit port, you connect ADDR31 to your LSB address line, then ADDR30, etc for as high as you have to go for the size of the memory.  Unused high order ADDR lines are not connected and the chip select will be set to define the memory size.

For a 16-bit port, you generally don't need ADDR31 and instead you rely on WE/BE[0:1] to select either of the two 8-bit halves.

On your board, you'd have a problem when you went to 16-bit mode as ADDR31 would always be low whenever you try to access the DATA[0:7] half i.e. you only use half of your memory.  You'd really want to shift all the ADDR lines up one bit.  I'm sure there is a way to do what you want but at the moment, I'm not seeing an easy way to connect that memory so you can use both options.

Regards,
Allan

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of murphynetuk
Sent: Wednesday, August 17, 2005 11:37 AM
To: [email protected]
Subject: [MPC500] ADDR []and DATA[] connections for 8 & 16 bit memory


Hi,
I want to layout my MPC555 board with the option of either 1 or 2 8-bit 
memory devices. With a single chip installed I would access it as 8- bit, with 2 chips I want to access them as a single 16-bit device (by 
approprate register entries). My question is how to layout the ADDR[] 
and DATA[] lines? I assume that DATA[0..7] would go to the single chip 
location and DATA[8..15] to the 2nd location. But what about ADDR[]? 
The reference manual is very unclear on this issue.
Regards, Ian





-----------------------------------------------------------
To learn more about Freescale Microcontrollers, please visit http://www.freescale.com/mcu


 
Yahoo! Groups Links



 




------------------------ Yahoo! Groups Sponsor --------------------~--> 
<font face=arial size=-1><a href="http://us.ard.yahoo.com/SIG=12h8kb2km/M=362329.6886308.7839368.1510227/D=groups/S=1706554205:TM/Y=YAHOO/EXP=1124313959/A=2894321/R=0/SIG=11dvsfulr/*http://youthnoise.com/page.php?page_id=1992
">Fair play? Video games influencing politics. Click and talk back!</a>.</font>
--------------------------------------------------------------------~-> 

-----------------------------------------------------------
To learn more about Freescale Microcontrollers, please visit
http://www.freescale.com/mcu


 
Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/MPC500/

<*> To unsubscribe from this group, send an email to:
    [email protected]

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/