Abstract: When designing microprocessor cards and memory subsystems cards for backplane-based systems, especially those cards containing data which must be retained during a hot-socket or unexpected power down situation, system designers must consider many issues and their impact on system design. Considerations include: Power monitoring for an orderly switchover to backup memory during power-down and a means to detect a "hot socket" event in time to facilitate the same switchover to backup supplies. This application note addresses these system level issues and discusses some of the devices available from Dallas Semiconductor, which can be used to implement solutions to these design issues.
Designing microprocessor cards and memory subsystem
cards for backplane-based systems challenges
system designers with problems not associated with
single motherboard systems. Power supply stabilization,
power supply monitoring, backup supply switching,
and maintaining memory or microprocessor states
when inserting or removing "hot" cards require that the
system designer take special care to isolate system
memory and microprocessor cards from transient
conditions generated by the backplane or by other system
cards. Dallas Semiconductor produces three product
families, prefabricated memory products, CPU
supervisory products, and integrated battery backup
products, which are particularly well suited for subsystem
card designs.
As an example let's say that you need to design a
memory subsystem with the following minimum requirements:
Board must have single-sided assembly.
2M x 16 of base memory required, with capabilities
to support up to an additional 2M x 16 of extended
memory, for a total of 4M x 16. Access times of 85
ns are required.
Memory must require no stand-alone refresh circuitry.
A backup power supply must be available to maintain the contents of the base memory and
extended memory on the card.
Power supply (5V) must be monitored both on the subsystem card as well as on the backplane card.
Switching from VCC to the secondary power supply must be automatically executed during a power-
down or power failure.
Switching from the backup power supply to VCC on power up must automatically occur once VCC
is in tolerance.
Switching system must be able to switch during a "hot card" situation without losing the contents of
memory, requiring a smooth transition from a high current active memory state to a low current backup
state.
The backup power supply must provide an advance warning of an impending out-of-tolerance condition on the
backup supply.
Memory must be write protected during a power failure to prevent it from becoming corrupted.
While the list seems lengthy, these requirements are an
essential minimum for many memory cards. Dallas
Semiconductor provides condensed, ready-made
solutions to these problems which simplify the card
design, number of piece parts required, and real estate
required for a solution. A block diagram of the required
functions follows:
Each of these system functions can be realized using a
condensed, off-the-shelf Dallas Semiconductor solution.
Let's begin with the memory requirements.
In order to provide a low-voltage, backup-compatible
memory, without the need for refresh circuitry, the best
choice is to use SRAM memory. The 85 ns memory
access requirement is available in SRAM as well. What
we also need, however, is a way to provide system
users with a convenient way of adding memory as they
need it. Most likely, the end user does not want to have
to add the memory SRAM by SRAM, or fumble with the
individual parts themselves. Dallas Semiconductor's
solution to this requirement is the DS2229. The DS2229
512K x 16-bit prefabricated memory Stik conforms to
the JEDEC 80-pin connection standard. The high density
word-wide configuration of the prefabricated
memory Stik provides the memory card with more
memory per surface area than surface-mounting the
individual SRAMs directly onto the memory card. The
modularity of a DS2229 makes it possible to provide the
memory card's user with maximum flexibility in adding
additional memory over the 2M x 16 minimum requirement.
Four DS2229's satisfy the minimum requirement,
with 80-pin JEDEC sockets on the card for up to four
more DS2229's. Since the DS2229's are also removeable
from the memory card, the card can be reconfigured
as necessary, or DS2229's from one card can be
used to upgrade the memory in other cards, or to provide
base memory for other cards, reducing the overall
cost of implementation for such a flexible memory subsystem.
512K X 16 80-Pin JEDEC Standard SIP Stik
With items 1, 2, and 3 of the system requirements met,
the next functions that can be easily consolidated into a
one-chip solution are the power switching functions.
The DS1336 is ideally suited for our system requirements
(see Figure 2). The device can switch between a
3V or 5V, 1.5A supply to a backup 3V supply, with a maximum
backup current of 4 mA. The DS1336 requires an
external notification of a power failure. Once the external
input is activated, the DS1336 immediately switches
from the primary power supply to the backup power supply,
in this case a lithium power source. Since the
DS1336 has two complemented power-fail inputs, Active-low PF
and PF , one can be used for the on-card power monitor,
and the other can be connected to a power-fail status
indicator on the backplane which can provide an
advanced warning of a power failure, before it reaches
the memory card.
What about the primary and backup power supply management
requirements? Again, Dallas Semiconductor
offers several condensed solutions which are user-configurable
to meet diverse system requirements.
Let's begin with the power monitoring requirements.
If we want to monitor the status of our on-board power
supply, backup supply, and also monitor the external
connections to the backplane, we need to be able to
monitor all of these supplies, and be able to use the output
signals to drive high current switching circuitry. Dallas
Semiconductor offers the DS1236 MicroManager,
which condenses all of the required functions into one
chip.
Figure 1. DS1236 MicroManager.
The DS1236 MicroManager monitors the primary
power supply VCC from the backplane, and the backup
lithium source. If VCC falls out of tolerance, the DS1236
provides two complemented power-fail signal outputs,
PF and PF, either of which can be used to signal to a
DS1336 to switch power from the primary supply to the
backup lithium energy source. The DS1236 also has a
user-definable voltage monitor, which in our memory
card implementation can be used to monitor the status
of the backup lithium source. In the implementation
shown below, if the backup lithium source's voltage falls
below 2.54V, the DS1236 will assert its /NMI\ output to
warn the memory controller or microprocessor that the
backup power supply has failed. This feature satisfies
requirement 9.
Figure 2. DS1336 Afterburner 16-Pin DIP or SOIC.
The last system requirement, that of write protecting
memory, is accomplished by controlling the chip enable
lines to the DS2229 prefabricated memory sticks. The
chip enable input from the memory controller is fed to
the DS1236, and is passed out as CEO during normal
VCC operating conditions. During an out-of-tolerance
condition, the DS1236 holds CEO high so that a write to
the memory with corrupted data can be prevented.
With the primary power supply monitoring requirements
satisfied, we still require a method to switch from the
higher active currents of the DS2229's to the very low
data retention currents required for maintaining the
memory in a standby state. The active current required
for the system is:
8 × (DS2229 ICC0) = 8 × 100 mA = 800 mA maximum
The data retention current required for the system is:
8 × (DS2229 ICC or standby) = 8 × 8 µA = 64 µA
The power requirements for maintaining the memory
contents in the absence of power is then:
2 years × 64 µA = 128 µA-years or 1122 mAh, so a minimum
of a 1.2 Amp-Hour lithium source will be required.
To switch between the high current and low current
requirements of the memory requires the ability to
switch between the 5V high current supply and 3V
backup supply, without dropping the current supply to
the memory below the minimum required to keep it
active. The DS1336 consolidates the required switching
functions into a single 16-pin SOIC device capable
of switching even more current than we require, up to
1.5A of active current. The DS1336 contains a series of
five power FETS which can each handle 300 mA, for a
combined capacity of 1.5A.
VCC from the primary power supply is tied to the five IN
pins of the DS1336, with the five OUT pins tied together
and to the VBAT01 pin. The VBAT01 pin of the DS1336
has a series diode to prevent VCC from charging the
backup lithium supply, which is connected to VBATIN.
When PF becomes active from the DS1236, indicating a
primary power failure on the memory card, the DS1336
switches power from the primary supply to the secondary
lithium supply within 100 ns. In order to monitor
power supplied by the backplane, the DS1336's other
power fail input, Active-low PF, can be connected to a Active-low PF signal on
the backplane, so that a backplane failure can also
cause the DS1336 to switch to the backup power
source.
For more information on high density nonvolatile memory applications and related subsystems, please call the
Dallas Semiconductor factory at (214) 450-0448.
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512K X 16 80-Pin JEDEC Standard SIP Stik
