MOTOROLA
MC68HC916X1
120
MC68HC916X1TS/D
8 Standby RAM Module
The standby RAM module (SRAM) provides 2 Kbytes of fast RAM that is especially useful for sys-
tem stacks and variable storage. The SRAM has a dedicated power supply pin so that memory con-
tent can be preserved when the MCU is powered down.
8.1 Overview
The SRAM module consists of a control register block that is located at a fixed range of addresses
in MCU address space, and a 2 Kbyte array of two bus cycle static RAM that can be mapped to any
2 Kbyte boundary in MCU address space. SRAM control registers are located at addresses
$YFFB00–YFFB08.
The module responds to program and data space accesses. Data can be read or written in bytes,
words, or long words. The RAM array must not be mapped so that array addresses overlap module
control register addresses, as overlap makes the registers inaccessible.
SRAM is powered by VDD in normal operation. During power-down, SRAM contents are maintained
by power from the VSTBY input. Power switching between sources is automatic.
8.2 SRAM Register Block
There are four SRAM control registers: the SRAM module configuration register (RAMMCR), the
SRAM test register (RAMTST), and the SRAM array base address registers (RAMBAH/RAMBAL).
8.3 SRAM Registers
SRAM responds to both program and data space accesses based on the value in the RASP field
in RAMMCR. This allows code to be executed from RAM.
Use RAMMCR to determine whether the RAM is in STOP mode or normal mode. RAMMCR can
determine in which space the array resides and also controls access to the base array registers.
Reads of unimplemented bits always return zeros. Writes do not affect unimplemented bits.
1. Y = M111, where M is the logic state of the module mapping (MM) bit in SCIMCR.
Table 63 SRAM Address Map
Address
15
8 7
0
$YFFB001
RAM MODULE CONFIGURATION REGISTER (RAMMCR)
$YFFB02
RAM TEST REGISTER (RAMTST)
$YFFB04
RAM ARRAY BASE ADDRESS REGISTER HIGH (RAMBAH)
$YFFB06
RAM ARRAY BASE ADDRESS REGISTER LOW (RAMBAL)
RAMMCR — RAM Module Configuration Register
$YFFB00
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
STOP
0
RLCK
0
RASP[1:0]
NOT USED
RESET:
1
0
1
0
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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