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© H. Heck 2008Section 2.21 Module 2:Transmission Lines Topic 2: Basic I/O Circuits OGI ECE564 Howard Heck

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.22 Where Are We? 1.Introduction 2.Transmission Line Basics 1.Transmission Line Theory 2.Basic I/O Circuits 3.Reflections 4.Parasitic Discontinuities 5.Modeling, Simulation, & Spice 6.Measurement: Basic Equipment 7.Measurement: Time Domain Reflectometry 3.Analysis Tools 4.Metrics & Methodology 5.Advanced Transmission Lines 6.Multi-Gb/s Signaling 7.Special Topics

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.23 Contents Push-Pull Transmitters (Drivers) Operation Modeling Open Drain Transmitters Operation Modeling Receivers Operation CMOS Receivers Modeling Driving a Transmission Line Summary References Appendix

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.24 Push-Pull Transmitters V SS V CC VssV CC V in V out V out [V] I out [mA] nMOS pMOS time [ns] V out [V]

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.25 Modeling Push-Pull Transmitters RSRS V out V SS VSVS R pmos R nmos V out V SS V CC Elements Transient voltage source Output resistance Known parasitics (usually capacitance) Thevenin Equivalent Asymmetric Switched Resistors Elements DC voltage source Output resistances (pull-up & pull down) Known parasitics (usually capacitance) Use the asymmetric model when your output transistors may have different impedances.

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.26 Open Drain Transmitters Characteristics (e.g. V-T curve) are specified into a standard load. V in V out I out V TT R TT V SS V CC V SS V CC V in V out V OL time [ns] V out [V] V out [V] I out [mA] nMOS I out [mA] pMOS I out [mA]

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.27 Model elements Time varying current source in parallel with resistance…or A switch in series with a resistance connected to ground Parasitics External termination supply ( V TT ) and resistor ( R TT ) Modeling Open Drain Transmitters I out V TT R TT V V in V out V TT R CC P1 N1 N2 N4 N3 Example: GTL circuit (1991).

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.28 Receivers The input buffer (receiver) is an amplifier. It takes the input signal and restores it to on-chip signal levels CLK D Q Receiver To Core Differential receivers require two input signals. A differential receiver can be converted to a single ended receiver by connecting one of the inputs to a reference signal ( V ref ). V in + V - V out V in V ref V out

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section 2.29 Single Ended ReceiversInverter Used for full swing signals (TTL, CMOS). Resistor and diodes are for ESD protection. Diodes “shunt” large voltage spikes into the supplies. Resistor limits the amount of current that is sunk in an ESD event. V CC Current limiting resistor (40 -200 ) ESD diodes Input pad (100mm x 100mm) Current Mirror Made by connecting one of the diff amp inputs to a stable reference input voltage. Often used in low swing designs, such as GTL+. V in V ref V out

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section Modeling Receivers As long as we’re using FETs (CMOS), the models that we use for interconnect analysis and simulation can be pretty simple. Gate oxide acts as an insulator, so the input transistor represents a high impedance to a static signal (usually an infinite resistance). However, the ESD protection circuitry and the gate of the receiver can contain significant capacitance (2-5 pF). So, our simplest receiver model is a capacitor. As we’ve seen, the package is often modeled using lumped R, L, and C elements C pkg R L C rcv Example

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section Driving a Transmission Line: Voltage Divider Analogy A transmission line presents an impedance ( ) to the output driver (transmitter). Push-pull example: As a result, the circuit behaves like a voltage divider: VSVS trtr RSRS Z0Z0 V i, I i VSVS trtr RSRS Z0Z0 ViVi IiIi + - [2.3.1] [2.3.2]

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section Load Line Analysis A more general method is load line analysis: Model the output buffer as an I - V curve. For linear transmitter model, Ohm’s law says slope = -1/ R S for a pull-up device From Ohm’s law, the transmission line has slope = 1/ Z 0 (rising edge). The initial current/voltage waves are defined by the intersection of the two curves. Load line analysis = graphical technique for solving simultaneous equations in I & V.

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section Load Line Example 5V 12.5 50 I V [V] I [mA] T-Line V=50I Driver V=5-12.5I 4.0V, 80 mA

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section More On Load Lines The voltage divider equation can be derived by solving the Ohm’s law equations for the output buffer and the transmission line. It is equivalent to a load line analysis for a low-high transition when there is no initial current flow. The slopes reverse for a high-to-low transition. We must also take into account initial voltage and current. So what? Why bother with load line analysis?

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section Summary Push-Pull and Open Drain drivers are both used in high speed designs. Receivers are amplifiers. We can use passive circuits to make approximate models for drivers and receivers. The output buffer and impedance (for a voltage mode driver) of the transmission line look like a voltage divider to the driven signal. Load line analysis provides a general method for predicting the initial signal amplitude on a transmission line. I will use the following terms synonymously: Transmitter Driver Output Buffer Receiver Input Buffer Transceiver I/O Buffer

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section References S. Hall, G. Hall, and J. McCall, High Speed Digital System Design, John Wiley & Sons, Inc. (Wiley Interscience), New York, W. Dally and J. Poulton, Digital Systems Engineering, Cambridge University Press, S. Dabral and T. Maloney, Basic ESD and I/O Design, John Wiley and Sons, New York, N. Wang, Digital MOS Integrated Circuits: Design for Applications, Prentice Hall, Englewood Cliffs, NJ, R. Poon, Computer Circuits Electrical Design, Prentice Hall, 1 st edition, H.B.Bakoglu, Circuits, Interconnections, and Packaging for VLSI, Addison Wesley, 1990.

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Basic I/O Circuits ECE 564 © H. Heck 2008 Section Appendix: Tri-State Transmitters Tri-state drivers are used for bi-directional and/or multi- drop buses. They are push-pull circuits with enable/disable logic. When multiple components can drive the same circuit, we need a way to disconnect inactive drivers. V out Output Enable Data OEDataV out 00X 01X When output is enabled (OE=1), the circuit acts as an inverter. When output is disabled (OE=0), both output transistors are cut-off.

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