Common Mode Considerations of the SuMMIT XTE (3/01)

Aeroflex UTMC Product Advisory
UT69151-XTE-ADV-001
Common Mode Considerations for the Aeroflex UTMC
Sµ
µMMITTM XTE Product Family
Table 1: Cross Reference of Applicable Products
SMD #
Device Type
Internal PIC
Number:
SµMMIT XTE (5V)
5962-94758
08
MM019
SµMMIT XTE (12V)
5962-94758
09
MM020
SµMMIT XTE (15V)
5962-94758
07
MM021
Product Name:
1.0 Overview
Aeroflex UTMC’s analysis of the SµMMIT XTE’s in-system, functional operation has identified some important common
mode related issues. These common mode issues are relevant for scenarios where the SµMMIT XTE receives signals from a
source that does not share power and ground planes with the SµMMIT XTE. In a system where the ground of the source electronics lacks the necessary low inductance current path (less than 5nH) to the SµMMIT XTE ground, read failures can occur.
The typical failure mode exhibits itself in the form of bad data reads or incorrect memory read cycle termination.
Table 1, identifies all applicable SµMMIT XTE product covered in this advisory. Previous versions of the SµMMIT XTE are
not at risk of the common mode failures discussed within this advisory because they are built with slower technology, and do
not have the same number of simultaneous switching I/O.
2.0 At Risk Systems
A system is at risk when the memory interface signals to the SµMMIT XTE do not share a power and ground plane with the
SµMMIT XTE. The applicable memory interface signals include:
1.
2.
3.
4.
5.
6.
Address [15:0],
Data [15:0],
ALE,
RD,
CS, and
DS
These memory interface signals must be common moded out in order to ensure proper functional operation of the SµMMIT
XTE particularly during memory reads. If any of these signals are electrically manipulated by a component, which does not
share the power and ground planes with the SµMMIT XTE, the system is at risk of common mode failures.
Some typical systems exhibiting failures include those where the memory reads across a backplane are initiated by a processor
on a different module than the SµMMIT XTE (Figure 1), and systems whose memory cycle activities are all contained on the
same module, but the SµMMIT XTE uses different power planes than the device performing memory accesses to the
SµMMIT XTE (Figure 2).
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Aeroflex UTMC Product Advisory
UT69151-XTE-ADV-001
Backplane
CPU Board
SµMMIT XTE Board
SµMMIT XTE
Address [15:0]
Processor
Data [15:0]
Memory Control
Signals
v(t) = L (di(t)/dt)
Figure 1. Memory Interface to SµMMIT XTE Over Backplane
Fully Integrated Circuit Card
With Split Power Planes
v(t) = L (di(t)/dt)
Processor Ground Plane
SµMMIT XTE Ground Plane
SµMMIT XTE
Processor
Address [15:0]
Data [15:0]
Memory
Control
Signals
Figure 2. Memory Interface to SµMMIT XTE on a Split Plane Circuit Card
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UT69151-XTE-ADV-001
2.1 Failure Mechanism
The primary cause of read failures in the systems described in section 2.0 above occur because of the high inductive current
path between the ground plane of the SµMMIT XTE and the ground plane of the device performing a memory read. This
inductive current path plays a major role during memory reads to the SµMMIT XTE because of the large simultaneous switching current induced by the SµMMIT XTE. The SµMMIT XTEs identified in Table 1 of this document consist of a 4 die,
multi-chip module architecture as shown by the functional block diagram in Figure 3 below.
MODE
STATUS
JTAG
INTERRUPTS
CHA
ADDRESS
TRANSCEIVER
CHA
SµMMIT
Protocol
Handler
DATA
MMU
INTERFACE
CONTROL
CHB
AUTO-INIT BUS
TRANSCEIVER
Memory
4k Bytes
CHB
REMOTE
TERMINAL
ADDRESS
Memory
64k Bytes
Figure 3. UT69151 SµMMIT XTE Block Diagram
During a memory read, the memory interface signals are directly interpreted by the Memory Management Unit (MMU)
located in the SµMMIT protocol die. The MMU determines whether the memory access is multiplexed or non-multiplexed, 8bits or 16-bits in size, and whether the access is being made to the SµMMIT Configuration Registers, the local 4kByte SRAM
or the external 64kByte SRAM. As a result, a number of devices, and buffers can be simultaneously switched from an “OFF”
or lower power state to an active high current state. An example would be reading a 16-bit word from the external 64kByte
memory. Such a scenario causes the SµMMIT protocol chip to activate 16 address buffers to the memory, bring the memory
from an idle to an active operating state, allow the high speed external memory to drive data to the SµMMIT, and enable the
16 data buffers from the SµMMIT to the host. All of this simultaneous activity causes a large current surge into the ground
plane via the SµMMIT XTE.
Typically, a very good decoupling scheme will provide an efficient current return path from the VSS pins to the V DD pins on
the SµMMIT XTE. However, due to the parasitics of the chip capacitors (e.g. effective series resistance) the current path is
not perfect, and the excess current must find an alternative return path through the system. Additionally, it is difficult to
decouple a complex circuit for all relevant current frequencies, further affecting the amount of excess current that is not efficiently routed through the decoupling capacitor.
For systems described in section 2.0, the excess current flow on the SµMMIT XTE’s power planes result in power and ground
bounce that is not well matched by power and ground bounce at the device reading the SµMMIT XTE. This is a direct result
of the large impedance between power planes used by the SµMMIT XTE and the power planes used by the component performing a read access to the SµMMIT XTE. The following example depicts the behavior seen in a system lacking common
mode.
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UT69151-XTE-ADV-001
Given:
1) System architecture as depicted in Figure 1.
2) Inductance between the ground planes on each board is 50nH
(Includes inductance through edge connectors on each board and the backplane)
3) The excess current sunk from the SµMMIT XTE into its ground plane is shown in Figure 4a.
Note: The information shown in this example is theoretical and not taken from empirical or factual data.
Then:
1) The potential between the SµMMIT XTE ground plane and the Processor ground plane is shown in Figure 4b.
{
Current Amplitude (mA)
0mA
for
t < 1ns
30(t - 1) mA
for
1 < t < 6 ns
-30(t - 11) mA
for
6 < t < 11 ns
0mA
for
t > 11 ns
i(t) =
150mA
100mA
50mA
-1ns 0ns
1ns
-50mA
2ns
3ns
4ns
5ns
6ns
7ns
8ns
9ns 10ns 11ns 12ns
Time (ns)
Figure 4a. Current Surge in Sµ
µMMIT XTE Ground Plane
Voltage Amplitude (Volts)
1.5V
1V
v(t) = L (di(t)/dt) = 50-9 * di(t)/dt
0.5V
-1ns
0ns
1ns
2ns
3ns
4ns
5ns
6ns
7ns
8ns
9ns 10ns 11ns 12ns
Time (ns)
-0.5V
-1V
-1.5V
Figure 4b. Potential Between the Sµ
µMMIT XTE Ground Plane and the Processor
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Using the above example, we can explain the reason for read failures to the SµMMIT XTE in systems lacking common mode
between the SµMMIT XTE and the reading device. Specifically, a large potential difference between the ground plane used
by the SµMMIT XTE and the ground plane used by the device performing a read results in the memory inputs to the SµMMIT
XTE being sensed at invalid levels. As a result, the SµMMIT XTE quickly reacts to the “effective” change in state, resulting
in bad data being latched at the SµMMIT XTE data outputs, or improper termination of the bus cycle as signaled by incorrect
behavior of the RDY pin on the SµMMIT XTE. Figure 5 shows a noise margin diagram that can be used in conjunction with
Table 2 to define the interface characteristics between the SµMMIT XTE and common buffer types.
VO Driving Device
VI Driving Device
VOHmin
High-Voltage Noise Margin
VIHmin
Uncertain
Region
VILmax
{
Low-Voltage Noise Margin
VOLmax
0V
Figure 5. Noise Margins
Table 2: Worst-Case Values of Primary Interfacing Parameters
Parameter
SµMMIT
XTE
UT54ACS164245S
Bus Transceiver
74HCMOS
AHC
74TTL
74LS
74AS
74ALS
VIHmin
2.2 V
3.85 V
3.5 V
3.85 V
2V
2V
2V
2V
VILmin
0.8 V
1.65 V
1V
1.65 V
0.8 V
0.8 V
0.8 V
0.8 V
V OHmin
2.4 V
3.8 V
4.9 V
3.8 V
2.4 V
2.7 V
2.7 V
2.7 V
VOLmax
0.4 V
0.4 V
0.1 V
0.44 V
0.4 V
0.4 V
0.4 V
0.4 V
IIHmax
10 µA
3 µA
1 µA
1 µA
40 µA
20 µA
200 µA
20 µA
IILmax
-10 µA
-1 µA
-1 µA
-1 µA
-1.6 mA
-400 µA
-2 mA
-100 µA
IOHmax
-4 mA
-8 mA
-4 mA
-8 mA
-400 µA
-400 µA
-2 mA
-400 µA
IOLmax
4 mA
8 mA
4 mA
8 mA
16 mA
8 mA
20 mA
4 mA
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Applying the data shown in Table 2 to the noise margin diagram in Figure 5, we can see that the SµMMIT XTE has typical
noise immunity of 0.4V typical on the low voltage side, and 0.2V to 2.7V of noise margin on the high voltage side depending
on the device used to drive the SµMMIT XTE. Extending this information to the ground bounce scenario described in Figure
4a and 4b, we notice when the SµMMIT XTE ground plane drops 1.5V below the reading device’s ground plane, then all low
signals driven to the SµMMIT XTE appear to be at least 1.5V. Referring to the noise margin diagram in Figure 5, an input of
1.5V to the SµMMIT XTE falls into the “Uncertain Region.” As a result, there is no guarantee how the SµMMIT XTE evaluates the state of signals in this region. If, for example, the input falling into this “Uncertain Region” is a control signal (i.e.
RD, CS, or DS), and the SµMMIT XTE determines the signal is high, it would prematurely terminate the bus cycle. If,
instead, the input falling into this “Uncertain Region” is an address input, and the SµMMIT XTE determined the address input
is high, it would fetch data from an incorrect address.
3.0 Techniques to Prevent Common Mode Failures
There are 3 primary methods to ensure that common mode failures do not occur when interfacing to the SµMMIT XTE. The
first, and most obvious, solution is to place the device initiating memory reads on the same module as the SµMMIT XTE and
ensure that both components share power and ground planes. The second preventative measure is to use schmitt triggered
input buffers on the SµMMIT XTE module. The third technique to minimize the risk of common mode failures is to latch the
SµMMIT XTE’s memory interface inputs once they are received at the SµMMIT XTE side of the system.
3.0.0 Local Integration to Prevent Common Mode Failures
The most certain method to ensure that your system does not exhibit any common mode failures is to design your system so
that all fast source and destination components share power and ground planes. It is important to distinguish the term “source”
refers to the devices that creates a signal, not the device that buffers/drives a signal to the destination. However, all devices in
the signal lane from the source to the destination should share the same power and ground planes. The objective here is to
reduce the inductive current path between the power/ground pins on the SµMMIT XTE and the power/ground pins on the
source component. By reducing the inductive current path between these pins, you ensure that both components have a very
small potential between the corresponding power/ground pins on each part. As long as this potential between parts resides
within the voltage noise margins depicted by Figure 5, then the SµMMIT XTE will always evaluate low and high inputs correctly.
3.0.1 Schmitt Triggered Input Buffers to Prevent Common Mode Failures
If a system can not include the source and destination components on the same module and allow them to share the same
power and ground planes, then the use of interface buffers offering schmitt triggered inputs like the UT54ACS164245S bus
transceiver shown in Table 2 will facilitate an increased noise margin and could filter false signal transitions caused by ground
bounce. The UT54ACS164245S has two major benefits. The first benefit is the large amount of noise margin for low signals.
Specifically, the UT54ACS164245S ensures that an input of 1.65V or less will be driven out at 0.4V or less. The second benefit of the UT54ACS164245S is the schmitt triggered input technology which requires glitches to cross the V IH or VIL threshold in order to change state. This effectively removes the “Uncertain Region” shown in the noise margin diagram in Figure 5.
3.0.2 Input Latching to Prevent Common Mode Failures
An alternative method to the schmitt trigger solution described in section 3.0.1 of this document is to insert data latches
between the SµMMIT XTE and the backplane interface. Using a data latch for the relevant memory control signals and
address bus inputs to the SµMMIT XTE ensures the ground bounce exhibited during a SµMMIT XTE read will be common
moded with the SµMMIT XTE and will not pass a signal glitch at the inputs of the latch. However, this solution is a little difficult to ensure that timing and clocking of the latches are sufficient to meet SµMMIT XTE timing requirements and do not
provide a false clock due to input glitches at the clock input to the latch.
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Modification Date: 3/28/01
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