1
Texas Instruments
Crossbar Switches
Ramzi Ammar
Advanced System Logic – Semiconductor Group
SCDA001A
July 1995
2
IMPORTANT NOTICE
Texas Instruments (TI) reserves the right to make changes to its products or to discontinue any
semiconductor product or service without notice, and advises its customers to obtain the latest
version of relevant information to verify, before placing orders, that the information being relied
on is current.
TI warrants performance of its semiconductor products and related software to the specifications
applicable at the time of sale in accordance with TI’s standard warranty . T esting and other quality
control techniques are utilized to the extent TI deems necessary to support this warranty.
Specific testing of all parameters of each device is not necessarily performed, except those
mandated by government requirements.
Certain applications using semiconductor products may involve potential risks of death,
personal injury, or severe property or environmental damage (“Critical Applications”).
TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED, INTENDED, AUTHORIZED, OR
WARRANTED TO BE SUITABLE FOR USE IN LIFE-SUPPORT APPLICATIONS, DEVICES
OR SYSTEMS OR OTHER CRITICAL APPLICATIONS.
Inclusion of TI products in such applications is understood to be fully at the risk of the customer .
Use of TI products in such applications requires the written approval of an appropriate TI officer .
Questions concerning potential risk applications should be directed to TI through a local SC
sales office.
In order to minimize risks associated with the customer ’s applications, adequate design and
operating safeguards should be provided by the customer to minimize inherent or procedural
hazards.
TI assumes no liability for applications assistance, customer product design, software
performance, or infringement of patents or services described herein. Nor does TI warrant or
represent that any license, either express or implied, is granted under any patent right, copyright,
mask work right, or other intellectual property right of TI covering or relating to any combination,
machine, or process in which such semiconductor products or services might be or are used.
Copyright 1995, Texas Instruments Incorporated
iii
Contents
Title Page
What Are Texas Instruments Crossbar Switches? 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bus Switches Provide 5-V to 3-V Translation When 3-V Supply Line Is Not Provided 3. . . . . . . . . . . . . . . . . . . . . . . . . .
Bus Switches Can Be Used to Replace Drivers and Transceivers in Bus Applications 3. . . . . . . . . . . . . . . . . . . . . . . . . . .
Bus Switches Convert TTL Logic to Hot-Car Insertion Capability 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
List of Illustrations
Figure Title Page
1 Output Voltage Versus Supply Voltage 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Output Voltage Versus Input Voltage 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 On-State Resistance Versus Input Voltage 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 5-V TTL to 3-V TTL Translator System 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 ACL Direction of Current Flow When VCC = 0 V 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 No ABT Current Flow When VCC = 0 V 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Hot-Card Insertion Application 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Power -Up High-Impedance State With CBT 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
What Are Texas Instruments Crossbar Switches?
Crossbar switches are high-speed bus-connect devices. Each switch consists of an N-channel MOS transistor driven
by a CMOS gate. When enabled, the N-channel transistor gate is pulled to VCC and the switch is on. These devices
have an on-state resistance of approximately 5 Ω and a propagation delay of 250 ps. They are capable of conducting
a current of 64 mA each. The transistor clamps the output at ≈ 1 V less than the gate potential, regardless of the level
at the input pin. This is one of the N-channel transistor characteristics (see Figures 1 and 2). Note the ≈ 1-V difference
between the gate (VCC) and the source (VO) at any point on the graph.
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
3 3.5 4.0 4.5 5.0
VCC – Supply Voltage – V
– Output Voltage – VVO
Figure 1. Output Voltage Versus Supply Voltage
2
0
1
2
3
4
5
012345
V
I
– Input Voltage – V
– Output Voltage – VVO
VCC = 5 V
Figure 2. Output Voltage Versus Input Voltage
The on-state resistance (ron) increases gradually with VI until VI approaches VCC – 1 V, where ron rapidly increases,
clamping VO at VCC – 1 V (see Figure 3). Also, by the nature of the N-channel transistor design, the input and output
terminals are fully isolated when the transistor is off. Leakage and capacitance are to ground and not between input
and output, which minimizes feedthrough when the transistor is off.
VI – Input Voltage – V
– On-State Resistance –ron
0
2
4
6
8
10
12
14
16
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Ω
Figure 3. On-State Resistance Versus Input Voltage
3
Bus Switches Provide 5-V to 3-V Translation When 3-V Supply Line Is Not Provided
These devices also can provide bidirectional 5-V to 3-V translation with minimal propagation delay or direction
control, using only a 5-V supply line and a diode. Figure 4 illustrates this application. A 4.3-V VCC can be created
by placing a diode between VCC and the switch. This causes gate voltage of 4.3 V due to the diode drop of
approximately 0.7 V. This drop, coupled with the gate-to-source drop of 1 V, brings VO to a maximum 3.3-V level
that can be used to drive a signal in a 3-V environment.
These devices consume very little current (ICC = 3 µA). This current is not satisfactory for the diode to operate. Using
a resistor from the cathode of the diode to GND allows more current from the supply voltage, causing the diode to
operate and to clamp at the specified 4.3 V (see Figure 4). The recommended value of the resistor is 1 KΩ or less.
VG = 4.3 V
VCC = 5 V
OE
5-V Memory
5-V I/O
3-V CPU
3-V Memory
3 V 5 V
3.3 V MAX
TTL-Level Signal
R
Figure 4. 5-V TTL to 3-V TTL Translator System
Bus Switches Can Be Used to Replace Drivers and Transceivers in Bus Applications
Bus switches introduce near-zero propagation delay. They can replace drivers and transceivers in systems in which
signal buffering is not required. They can be used in a multiprocessor system as a fast bus connect, or they can be
used as a bus-exchange switch for crossbar systems, ping-pong memory connect, or bus-byte swap. These devices
also can replace relays that are used in automated test equipment (ATE) to connect or disconnect load resistors in
negligible time with the same low on-state resistance and without relay-reliability problems.
4
Bus Switches Convert TTL Logic to Hot-Card Insertion Capability
This application is used mostly in systems that require hot-card insertion or removal of cards without disturbing or
loading down the bus. These systems are designed to run continuously and cannot be shut down for any reason, such
as telephone switches, manufacturing controls, real-time transaction systems, and airline-reservation networks.
These systems/cards use some logic families like ACL, HCMOS, etc., which do not provide isolation from the bus
when power is partially removed, causing system error. Also, connectors are designed so that the ground pins are
connected first, followed by the signal pins, then VCC last. In this condition, the existing logic must ensure that the
I/O signals do not disturb or load down the bus. This assurance cannot be achieved using CMOS logic since it contains
P-channel transistors that provide an inherent diode between the I/O pins and VCC. The diode is forward biased when
driven above VCC (see Figure 5). In a situation where VCC is disconnected, these diodes are capable of pulling the
system bus to approximately one diode drop above ground, leaving the bus disturbed.
VCC = 0 V
Input Output
Figure 5. ACL Direction of Current Flow When VCC = 0 V
Another issue to consider is that, when VCC is ramping but still below the device-operating voltage, the logic should
ensure that the outputs are in the high-impedance state and that the bus is totally isolated until the card is ready for
operation. Finally , the capacitance of the card must be seen by the system bus as low as possible so that when the card
is inserted and the capacitance is charged up, disturbance or bus error does not occur.
There are two solutions to this problem; one is to use Texas Instruments BiCMOS technology (BCT) or advanced
BiCMOS technology (ABT) families, since both ensure the input and output to be off when V CC is removed due to
the absence of the clamping diodes to VCC (see Figure 6). They also provide an active circuit that ensures the output
to be in the high-impedance state during part of the VCC power up or power down.
Input Output
No Path to VCC
Figure 6. No ABT Current Flow When VCC = 0 V
5
The second solution is to use the T exas Instruments CBT family. This can be done by placing the switch between the
card logic and the connector to serve as an isolator when power is removed. The switch uses an n channel that prevents
the current from flowing into the switch when powered down (see Figure 7). One device in particular, the
SN74CBT6800, is designed specifically for hot-card insertion. It has a built-in channel pullup tied to a bias voltage
(BIASV) that is provided to ensure power up with the buses not connected. Other devices can be used in the same
manner , however, to ensure the high-impedance state during power up or power down. The enable pins of the switch
should be tied to VCC through a pullup resistor. The minimum value of the resistor is determined by the
current-sinking capability of the driver (see Figure 8).
CPU
Memory
Connector CBT Logic I/O
Plug-In Card
Bus
Figure 7. Hot-Card Insertion Application
IR
VCC
Control Circuit
VCC
Enable Pin (active low)
CBT Device
IOL†
†IOL > IR, so the control signal can override the pullup resistor.
Figure 8. Power-Up High-Impedance State With CBT
Conclusion
Texas Instruments crossbar switches can be used in several applications. Although they are simple N-channel
transistors, they are capable of providing several important bus functions, such as hot-card insertion, near-zero-delay
communication, 5-V to 3-V translation, and memory management in multiprocessor environments.
