Engineer To Engineer Note EE-103 Technical Notes on using Analog Devices' DSP components and development tools Phone: (800) ANALOG-D, FAX: (781) 461-3010, EMAIL: dsp.support@analog.com, FTP: ftp.analog.com, WEB: www.analog.com/dsp Performing Level Conversion Between 5v and 3.3v ICs Q: I am currently using the ADSP-2187L in my system and I want to interface a 5v FPGA to the DSP. However, I have concerns about potential interface issues caused by differing logic standards. Do you have any information on how to interface ICs that operate at different voltage/logic levels? A: There are a number of processors produced by Analog Devices that operate at 3.3v. The 3.3v fixed point processors include the ADSP-2183, ADSP-2184L, ADSP-2185L, ADSP-2186L and ADSP-2187L. Our 3.3v floating point processors are the ADSP-21060L, ADSP-21061L, ADSP21062L, and the ADSP-21065L. Additionally, all of our "M" family processors are capable of supporting 3.3v I/O standards. However, none of these devices can tolerate greater than Vcc+0.5v on their inputs. Therefore, additional logic is necessary to connect our 3.3v and "M" family DSPs to 5v peripherals. One simple way of solving this problem is by adding a Bus Switch (QuickSwitch) to the system, which adds voltage tolerance between two ICs. Bus Switches are essentially bus voltage level translators that are used to shunt the maximum voltage that is received by an IC. A typical transfer function of a Bus Switch is given below (Figure 1): V out Copyright 1999, Analog Devices, Inc. All rights reserved. Analog Devices assumes no responsibility for customer product design or the use or application of customers' products or for any infringements of patents or rights of others which may result from Analog Devices assistance. All trademarks and logos are property of their respective holders. Information furnished by Analog Devices Applications and Development Tools Engineers is believed to be accurate and reliable, however no responsibility is assumed by Analog Devices regarding the technical accuracy of the content provided in all Analog Devices' Engineer-to-Engineer Notes. 5 4 3 2 1 0 0 1.65 3.3 5 Vin Figure 1: Voltage I/O Characteristic of a Bus Switch (Vcc = 4.3v) The transfer function of a Bus Switch is as follows: Vout = Vin when Vin<Vcc-1 Vout = Vcc-1 when Vin>Vcc-1 (1) (2) In this example, with Vcc set to 4.3v, the Bus Switch will pass Vin to Vout until Vin equals 3.3v. Once Vin rises higher than 3.3v, Vout continues to hold at 3.3v. Therefore, placing a Bus Switch between a 5v and 3.3v device will effectively remove voltage incompatibility issues. Bus Switches are bi-directional ICs; therefore, no additional routing logic is needed for the connection of bi-directional buses. When the 3.3v DSP transmits information to the 5v device, its output voltage is not shunted or reduced by the Bus Switch. A simple way of providing 4.3v to the Bus Switch is to place a diode between the 5v system voltage and the Vcc pin on the Bus Switch. The voltage drop across the diode will force Vcc to approximately 4.3v. a Once concern when adding glue logic to a system is the potential added propagation delay. Bus Switches have very low propagation delay, on the order of 0.25ns. Therefore, they can safely be used in a wide range of timing sensitive applications and systems. Bus Switches are manufactured by a number of semiconductor companies, such as Pericom and Quality Semiconductor. For more information on the operation of these devices, please refer to the following resources: www.pericom.com www.qualitysemi.com Bus Switches typically have a part number of 3384. Therefore, searching each companies' web site for this part number brings up datasheets and application notes fully describing their proper usage. For more information on the electrical characteristics of the DSPs manufactured by Analog Devices, please refer to the appropriate DSP data sheet. EE-103 Page 2 Technical Notes on using Analog Devices' DSP components and development tools Phone: (800) ANALOG-D, FAX: (781) 461-3010, EMAIL: dsp.support@analog.com, FTP: ftp.analog.com, WEB: www.analog.com/dsp