Design Examples of On Board Dual Supply Voltage Logic Translators

AND8336
Design Examples of On Board
Dual Supply Voltage Logic
Translators
Prepared by: Jim Lepkowski
ON Semiconductor
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Introduction
Logic translators can be used to connect ICs together that
are located on the same PCB and use different supply
voltages. Figure 1 lists popular applications that use dual
supply voltage translators to interface a microprocessor and
peripheral IC. The following design examples will be
discussed in this document:
• I2C™ Bus
− SMBus
− PMBus
• SPI Bus
• Memory Mapped I/O
• UARTs
• USB Ports
Figure 1. Dual Power Supply Translators Connect
ICs Together in Mixed Voltage Systems by Shifting
the Logic Levels of the Control and Data Signals
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July, 2008 − Rev. 0
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I2C Bus
Figure 2. Open-Drain Autosense Voltage Translators Connect a Processor to I2C Bus Peripherals that have
Different Operating Voltages
The Inter-Integrated Circuit (I2C) bus was developed by
Philips (NXP) in the early 1980s and has developed into an
industry standard protocol. Figure 2 provides a design
example of a microprocessor system that incorporates the
I2C bus. The main advantage of I2C is that only two I/O lines
are required. A read or write operation is determined by the
R/W bit, and microprocessor direction and address pins are
not required. In addition, data is received and transmitted
independent of the microprocessor’s control routine. A
read/write operation receives/transmits data to the
processor’s control routine only after the data buffer is filled.
Figure 3 and reference [1] provide details on the I2C
open-drain bus protocol.
Figure 3. The I2C Serial Communication Frame Consists of Serial Clock (SCK) and Serial Data (SDA) Signal Lines
Several popular communication buses have emerged that
are similar to I2C to serve target applications. The System
Management Bus (SMBus), shown in Figure 4, is widely
used in PC motherboard applications such as thermal
management systems that monitor the microcontroller and
PCB temperatures, in addition to controlling the cooling fan.
The Power Management Bus (PMBus), shown in Figure 5,
is an emerging standard in power delivery systems. In
contrast, the I2C bus is the preferred bus for general purpose
microcontroller applications. The main advantage of the
SMBus is the bus recovery feature that resets the open-drain
signals if they are at a logic “0” state for more than 35 ms.
An overview of the attributes of the I2C and SMBus
protocols is shown in Table 1.
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Figure 4. The SMBus is Used to Transfer Control Information and Implement a Clock−Skipping Mode to Reduce
the Microcontroller’s Temperature
Figure 5. The PMBus is Often Used in Point of Load (POL) Power Supplies to Provide Control and Status
Information to a Power Monitor Microcontroller
Table 1. ATTRIBUTES OF THE I2C AND SMBUS SERIAL COMMUNICATION PROTOCOLS
I2C
Parameter
SMBus
Timeout / Data Recovery
No
Yes (35 ms)
Min. Clock Speed
DC
10 KHz
Max. Clock Speed
100 KHz / 400 KHz / 3.4 MHz
Standard / Fast / High-Speed Mode
100 KHz
VH
0.7 * VDD, 3.0 V (Fixed)
2.1 V
VL
0.3 * VDD, 1.5 V (Fixed)
0.8 V
IMAX
3 mA
350 mA
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SPI Bus
Figure 6. SPI Consists of a Chip Select (CS), System Clock (SCK), Serial Data Output (SDO) and Serial Data Input
(SDI) Signals
The Serial Peripheral Interface (SPI) bus, shown in
Figure 6, is a synchronous serial communication protocol
that can simultaneously receive and transmit data. SPI is a
popular protocol developed from the Motorola (Freescale)
Master Output Slave Input (MOSI) and Master Input Slave
Output (MISO) microprocessor bus. SPI offers a higher data
transfer rate than I2C because there is no maximum clock
frequency specification. A negative feature of SPI is that the
protocol requires either a chip select line per peripheral or
address decoding.
Figure 7. The Three Input / One Output Circuit Topology of the NLSV4T3144 Matches the I/O Structure of the Four
Wire SPI Bus
Both uni-directional and autosense voltage translators can
be used to interface a microprocessor and SPI peripheral, as
shown in Figures 7 and 8. The I/O pin locations of the
NLSV4T3144 and NLSX3014 accommodate a straight feed
through design, which is a convenient PCB layout feature.
Figure 9 shows an autosense translator can be used in SPI
ports that use a single bi-directional data line.
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Figure 8. The Autosense Translator’s I/O Pins Can Function as Either an Input or Output, Which is an Advantage
in SPI Translator Applications
Figure 9. Autosense Translators Can be Used for SPI Ports that Combine the Input (SDI) and Output (SDO) Data
Lines into a Single Bi-directional Signal (SI/O)
Memory Mapped I/O
IC via a Read/Write statement. The microcontroller address
pins select or ’map’ multiple peripheral devices to a single
data bus. Figures 10 and 11 provide examples of memory
map I/O translator applications.
Memory Mapped I/O circuits typically use bi-directional
with direction pin translators to connect multiple peripheral
ICs to a single microcontroller. The directional pin
determines if data is received or transmitted to a peripheral
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Figure 10. Memory Mapped I/O Translators are Often Used with Relatively High Pin Count Microprocessors that
Need Fast Data Transfers
Memory Mapped I/O uses a parallel data format.
Therefore the transfer rate is proportional to the number of
data bits. In addition, the communication speed is equal to
the duration of the Read/Write instruction cycle time of the
microcontroller and peripherals; thus, the control program
is delayed by slow I/O devices. A disadvantage of Memory
Mapped I/O is that address lines are used to generate the chip
enables of the peripheral ICs because a relatively large
number of microcontroller I/O pins are required. For
example, the circuit shown in Figure 10 requires 11
microprocessor I/O lines (8 data, 1 Read/Write, 2 address
lines). In contrast, the I2C bus uses only 2 I/O lines (SDA,
SCK).
Figure 11. The MC74LVX4245 is a Dual Supply Bi-Directional with Direction Pin Translator that can be Used to
Interface a 5 V I/O Bus to a 3.3 V Microprocessor
UARTs
transfers data between a control and audio processor. A logic
translator is needed in this example because the audio IC
operates at a higher voltage than the control processor in
order to produce a detectable audio signal. In addition,
voltage translators are often required for UARTs in RS-232,
RS-422 and RS-485 applications.
A universal, asynchronous receiver / transmitter (UART)
communications port can be used to transfer data between
the processor and a peripheral device. UART ports are
included in most microprocessors and are also available in
stand alone ICs. Figure 12 shows a circuit where the UART
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Figure 12. Voltage Translators are Often used with UARTs to Interface a Low Voltage Processor to a Higher
Operating Voltage Analog IC, Such as an Audio Processor
USB Ports
autosense and uni-directional translator, respectively.
Voltage translators offer cost savings over standard USB
transceivers.
Dual supply voltage translators can be used to shift the
voltage levels of a Universal Serial Bus (USB) signal.
Figures 13 and 14 provide design examples using an
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Figure 13. An Autosense Translator Can be used to Increase the Voltage of a Low Voltage Microcontroller that
has an Integrated USB Port
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Figure 14. A Dual Two Channel Uni-Directional Translator Can Function as a Bi-Directional USB Transceiver
Bibliography
1. “UM10204: I2C Bus Specification and User
Manual”, NXP Semiconductor, 2007.
Industry Websites for Further Information
• Power Management Bus (PMBus), www.pmbus.org.
• Systems Management Bus (SMBus), www.smbus.org.
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APPENDIX I: ON DUAL POWER SUPPLY AUTOSENSE VOLTAGE TRANSLATORS
Autosense Bi-Directional Translators
(Push-Pull Output)
Autosense Bi-Directional Translators
(Open-Drain Output)
Block Diagram
Attributes
Trade-Offs
Applications
ON Products
(I/O Channels /
Package)
•
•
•
High Data Rate
High Output Drive
Flexible PCB Design
•
•
•
•
Modest Output Current
•
Modest Bandwidth
•
•
•
•
SPI
•
•
•
•
•
•
I2C, SMBus, PMBus
•
•
NLSX3373 (2-bit, UDFN-8)
•
•
•
•
•
Low Power Consumption
UARTs
USB Ports
GPIO
NLSX3012 (2-bit, UDFN-8)
NLSX3014 (4-bit, UQFN-12)
NLSX3013 (8-bit, CSP-20)
NLSX3018 (8-bit, UDFN-20)
NLSX4014 (4-bit, UQFN-12)
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Low Power Consumption
Flexible PCB Design
SIM / SDIO Cards
Display Modules
HDMI
1-Wire Bus™
GPIO
NLSX3378 (4-bit, CSP-12)
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APPENDIX II: ON DUAL POWER SUPPLY UNI-DIRECTIONAL AND BI-DIRECTIONAL WITH DIRECTIONAL PIN
VOLTAGE TRANSLATORS
Bi-Directional with Directional Pin
Translators
Uni-Directional Translators
OE
T/R
A1
Block Diagram
B1
A2
B2
Attributes
•
•
High Data Rate
High Data Rate
Low Power Consumption
•
•
•
Fixed Input & Output Pins
•
All I/O Lines Configured for Either A-to-B or
B-to-A Single Direction Translation
•
Control Pin Limits Usability in Pin Sensitive
Portable Applications
•
•
Memory Mapped I/O
•
MC74LVXC4245 (8-bit, SOIC-24,
TSSOP-24)
•
MC74LVXC3245 (8-bit, SOIC-24,
TSSOP-24)
Trade-Offs
Applications
ON Products
(I/O Channels /
Package)
•
•
•
SPI
•
•
•
•
•
•
NLSV1T34 (1-bit, ULLGA-6)
USB Ports
High Output Drive
GPIO
GPIO
NLSV1T240 / 244 (1-bit, UDFN-6)
NLSV2T240 / 244 (2-bit, UDFN-8)
NLSV4T240 / 244 (4-bit, UQFN-12)
NLSV4T3234 (4-bit, CSP-11)
NLSV8T240 / 244 (8-bit, UDFN-20)
I2C Bus is a registered trademark of NXP / Philips Semiconductor.
1−Wire Bus is a registered trademark of Maxim / Dallas Semiconductor.
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