fax id: 3613 Using CY7B991 (RoboClockTM), CY7B9911 (RoboClock+), and CY7B9910 (Robo Jr.) in 3.3-Volt Environments Introduction The RoboClockTM family of low skew clock buffers includes six products listed in Table 1. Table 1. RoboClock Family Cypress Part No. Features CY7B991 15-80 MHz outputs with /2, /4, invert and programmable skew. TTL output levels. CY7B992 15-80 MHz outputs with /2, /4, invert and programmable skew. CMOS output levels. CY7B9910 15-80 MHz outputs. TTL output levels. CY7B9920 15-80 MHz outputs. CMOS output levels. CY7B9911 15-100 MHz outputs with /2, /4, invert and programmable skew. TTL output levels. CY7B991V 15-80 MHz output with /2, /4, invert and programmable skew. LVTTL output levels. 3.3V VCC. ation. Tying the VCCN pins to a voltage other than +5V may damage RoboClock. RoboClock's Output Buffers The output buffers on RoboClock can drive transmission lines down to 50. The ability to drive low-impedance transmission lines is a result of RoboClock's high current drive output buffers. The voltage-current relationships (or V-I curves) of RoboClock's outputs are shown in Figures 1 and 2. Note in Figures 1 and 2 current polarity is defined as positive when sinking (i.e., current is flowing into the buffer), and negative when sourcing (i.e., current is flowing out of the buffer). For 3.3V applications, the CY7B991V (Low Voltage Programmable Skew Clock Buffer) is ideal. It is a true 3.3V device with all the same functionality of the CY7B991/2. The rest of the RoboClock family is a 5V product line requiring all of the power supply pins to be connected to a single +5V supply. However, the 5V RoboClock family can still operate in mixed 5V/3V applications. Although the 5V RoboClock is not as ideal as the CY7B991V in 3.3V applications, it is still possible to make the outputs 3.3V compliant. the voltage levels on RoboClock's 5V TTL outputs may not be tolerable by the inputs of strict 3.3V LVTTL products. However, by using the termination network recommended in the data sheet (see Figure 4) it is possible to make the TTL level RoboClock (CY7B991, CY7B9911 and CY7B9910) outputs 3.3V-compliant. The CMOS level RoboClocks (CY7B992 and CY7B9920) cannot be made 3.3V-compliant due to their design which achieves rail-to-rail output swings. All the following sections pertain to the CY7B991, CY7B9910, and CY7B9911. Figure 1. RoboClock Output Buffer V-I Curve, Output = High RoboClock's Power Pins All RoboClock products have six power supply pins separated into two groups. The first group consists of two pins labeled VCCQ. These pins supply power to the logic and Phase Locked Loop (PLL) circuitry. The second group, labeled VCCN, consists of four pins. Each VCCN is a dedicated power supply pin for a particular clock output pair (1Qx, 2Qx, 3Qx, and 4Qx). Both the VCCQ and VCCN pins must be connected to a +5V power supply (except for the CY7B991V). It is not possible to operate RoboClock with the V CCN pins at 3.3V in the hopes of limiting the output buffers to 3.3V tolerant oper- Cypress Semiconductor Corporation * 3901 North First Street * San Jose * CA 95134 * 408-943-2600 May 1996 - Revised June 26, 1998 Using RoboClock in 3.3-Volt Environments signs). Most PCB transmission lines are 50, requiring a 50 termination. 50 Load for 3.3V Compliance A 50 characteristic impedance transmission line requires a 50 termination in order to prevent voltage reflections. However, the actual termination is not as simple as using a 50 pull-down resistor. RoboClock's data sheet switching characteristics (tSKEWPR, tSKEW1-4, tDEV, tODCV, tPWH, tPWL, tORISE, and tOFALL) are optimized when terminating to a voltage of 2.06V. Therefore, the best RoboClock output termination provides for a 50 equivalent load, but also sets the termination voltage to 2.06V. To verify that a RoboClock output terminated to a specific Thevenin resistance and voltage actually meets the JEDEC 3.3V requirements, Equation 1 must be solved iteratively, with the result compared against the V-I curve of Figure 1. Figure 2. RoboClock Output Buffer V-I Curve, Output = Low V Output - V Termination ----------------------------------------------------- = I Output Z Thevenin Eq. 1 V Output - 2.06 V ---------------------------------------- = IOutput 50 Eq. 2 With a 50 to 2.06V termination, Equation 2 can be solved using an iterative process (i.e., choosing a VOutput and solving for IOutput, until the VOutput and IOutput results agree with the V-I curve shown in Figure 1 since we are concerned with limiting the maximum output HIGH voltage) giving a solution of VOutput= 3.25V and IOutput=23.8 mA. The output buffers are designed to operate in systems with terminated transmission lines. By modifying the termination network, the output buffers can be loaded down (i.e., required to supply more current) resulting in a reduction in their voltage swing. In other words, RoboClock's outputs can be modified for 3.3V tolerant operation by choosing the correct termination network. With the termination chosen to meet 3.3V voltage requirements, the actual resistor values can be found using the circuit and equations shown in Figure 3. 3.3V-Compliant RoboClock Outputs VDD The JEDEC standard JESD8-A "Interface Standard for Nominal 3V/3.3V Supply Digital Integrated Circuits" defines the voltage levels for 3V- and 3.3V-compliant signaling. For a 3.3V-compliant digital input, the allowable voltage levels, as indicated in JESD8-A, are shown in Table 2. Table 2. JEDEC 3.3V Input Specifications, VDD=3.3V Parameter Min. Max Units VIH High-Level Input Voltage 2.0 VDD+0.3 V VIL Low-Level Input Voltage -0.3 0.8 V R1 1 1 1 ------- + ------- = ----------50 R1 R2 Eq. 3 R2 R2 -------------------x V DD = 2.06 V R1 + R2 Eq. 4 Figure 3. Choosing Termination Resistor Values Solving Equations 3 and 4 for VDD values of 5V and 3.3V, and choosing standard resistor values, gives the two termination networks shown in Figure 5. The outputs of the CY7B991V naturally comply to this standard. To achieve 3.3V compliant output levels, RoboClock's output buffers must be limited to swing no higher than 3.6V (3.3V+0.3V). From the curve trace in Figure 1, the output buffers can source 6.63 mA of current at 3.6V. Therefore, the appropriate termination network needed to achieve 3.3V operation is 3.6V/6.63mA560. The simplest 3.3V-compliant RoboClock design could use a 560 pull-down resistor on the RoboClock outputs. However, since transmission lines should be terminated to their characteristic impedance, a 560 termination resistor on the output of RoboClock would require use of a 560 transmission line (uncommon among printed circuit board de2 Using RoboClock in 3.3-Volt Environments Conclusion +5V +3.3V 130 82 91 130 Figure 4. Typical 50 Terminations to 5V and 3.3V for 3.3V-Compliant RoboClock Outputs The CY7B991V is a true 3.3V device and should be used in 3.3V applications whenever possible. The rest of the RoboClock family of clock drivers were designed for 5V environments. With the advent of 3.3V systems, many designs require "3.3V tolerant" waveforms. This application note has shown how to use the V-I relationships of the CY7B991, CY7B9910 and CY7B9911 outputs, and the characteristic impedance of a transmission line, to choose an appropriate termination network which guarantees that the outputs remain within 3.3V voltage swing specifications. Using these techniques, the test load from the RoboClock data sheet proves to be an appropriate termination to achieve 3.3V compliance. Note, the termination network for VDD=5V is actually the recommended output load as indicated in the RoboClock data sheet. With these termination networks, the RoboClock output waveform conforms to 3.3V specifications as shown in Figure 5. 3V Figure 5. RoboClock Output with 50 Load, VDD=5V RoboClock is a trademark of Cypress Semiconductor Corporation. (c) Cypress Semiconductor Corporation, 1998. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use of any circuitry other than circuitry embodied in a Cypress Semiconductor product. Nor does it convey or imply any license under patent or other rights. Cypress Semiconductor does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress Semiconductor products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress Semiconductor against all charges.