MPC555 / MPC556
STATIC RANDOM ACCESS MEMORY (SRAM)
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
20-1
SECTION 20
STATIC RANDOM ACCESS MEMORY (SRAM)
The MPC555 / MPC556 contains two static random access memory (SRAM) modules:
a 16-Kbyte module and a 10-Kbyte module. The SRAM modules provide the micro-
controller unit (MCU) with fast (one cycle access), general-purpose memory. The
SRAM can be read or written as either bytes, half words, or words.
Each SRAM module is built with a series of 4-Kbyte blocks and occupies a continuous
block of memory. For a RAM size array block of less than 4 Kbytes (e.g., the 2-Kbyte
array in the 10-Kbyte SRAM module), the remaining 2 Kbytes are unimplemented and
unusable.
The SRAM modules are accessible to the CPU and other bus masters via the L-bus
on the CPU chip. To improve access time, each SRAM module resides on a separate
bus interface unit (BIU). Each BIU has its own module control register.
20.1 Features
• One-cycle access
• Byte, half-word, or word read/write accesses
• Individual protection control bits provided for 4-Kbyte block boundaries
— read only region
— data only region
— user/supervisor
• Two-cycle access for power savings
• Low power standby operation for data retention
— V
DDI
= 0, no read/writes to the SRAM; VDDSRAM = 3.3 V to retain data
• Supports pipelining
20.2 Block Diagram
shows the major components of an SRAM module.
Figure 20-1 SRAM Block Diagram
SRAM Array
BIU
(Maximum 16 Kbytes)
Keep-Alive Power
L-bus
Power Gone
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..