martes, 19 de junio de 2012

How to convert TTL (5V) to LVTTL (3.3V)

When you are working with arduino, sometimes it is necessary to convert from TTL to LVTTL. The aim of this paper is to describe how to perform the conversion from TTL logic (ej. arduino mega 2560) to LVTTL logic (GPS receiver or other devices)

Conversion from TTL to LVTTL is very common when you need interface with different devices. After my latest acquisition (arduino MEGA 2560) and diving on the internet, I realized that many people working with arduino make questions about how to convert from TTL to LVTTL so, I decided to show one way to do it.

Different technologies  are possible for this logic conversion depending on speed, voltage, control inputs, output current, etc. In this paper, I will to describe the CB3T (Low-Voltage Translator Bus
Switch Family) technology from Texas Instrument. 

The 2.5-V/3.3-V voltage translator family, CB3T, provides a high-performance, low-power voltage translation interface solution for a variety of applications. This family is the only pure solution available with the capability to translate from 5-V down to 2.5-V when signal buffering (current drive) is not required with signal speed up to 100 MHz.

For further information, the figure 1 shows a table with a comparison of different switching standards. In our case,  "D" is the Driver and "R" is the receiver. No translation is necessary to transfer signals from the 3-V bus to the 5-V (TTL) bus, since the VOH level from the CB3T switch is greater than the required VIL of the 5-V (TTL) devices connected to the 5-V bus (see figure 2)

TTL, LVTTL compatibilidad
Figure 1 - Logic Standards Compatibility

TTL-LVTTL niveles lógicos
Figure 2 - TTL-LVTTL Logic Window 

The figure 3 shows a CB3T bus switch interfacing a 3-V bus with a 5-V (TTL) bus. The CB3T device down-translates the signals from the 5-V bus to 3-V levels.
Conversion TTL-LVTTL
Figure 3 - Conversion TTL-LVTTL using CB3T devices

The figure 4 shows the interconnection block diagram to interface TTL and LVTTL devices. The 8-bit SN37CBT3245 switch was used.
SN74CB3T3245 Ejemplo de inerconexión
Figure 4 SN74CB3T3245 Interconnection Diagram Example

Summarizing by means of CB3T devices, TTL and LVTTL can be connected without any problem of interconnectivity from both sides.

Texas Instruments Logic Guide

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