Interfacing the 3-V MSP430™ MCUs to 5-V Circuits

This application report describes the interface of a 3-V MSP430™ microcontroller (MCU) to a circuit with a supply of 5 V or higher. This report explains input, output, and I/O interfaces and provides worst-case design equations where necessary. This report also describes some simple power supplies that generate both voltages.

Trademarks

MSP430, LinCMOS are registered trademarks of Texas Instruments.

All other trademarks are the property of their respective owners.

1 Introduction

The MSP430 Sensing and Measurement MCUs support a supply voltage range of 1.8 V to 3.6 V. This is due to the manufacturing process used and has the advantage of drawing even less current than with a 5-V supply.

If an interface to a 5-V system or to a system with an even higher voltage is necessary, it can result in difficulties. This application report explains 5-V interfaces for the inputs, outputs, and I/Os of an MSP430 MCU. Figure 1 shows examples of input, output, and I/O interfaces. The shaded boxes in the figure are the topic of this application report.

NOTE

Throughout this document, the term MSP430 MCU stands for the members of the different MSP430 families. Refer to the device-specific data sheets for parameter specifications.

The given formulas for the external supply voltage V_(sys) also can be used for higher voltages than 5 V. The equation are useful for any external voltage; for example, V_(sys) = 12 V.

Figure 1. Interfaces Between the 3-V MSP430 MCU and 5-V Systems

With the worst-case equations, the following simplifications are used for the calculation with small values of a_x (like for the tolerance p):

Equation 1.

\frac{1}{1 + a_{x}} \approx (1 - a_{x}) \frac{1}{1 - a_{x}} \approx (1 + a_{x}) \frac{1 + a_{x}}{1 - a_{x}} \approx (1 + 2 a_{x}) \frac{1 - a_{x}}{1 + a_{x}} \approx (1 - 2 a_{x})

$\frac{1}{1 + a_{x}} \approx (1 - a_{x}) \frac{1}{1 - a_{x}} \approx (1 + a_{x}) \frac{1 + a_{x}}{1 - a_{x}} \approx (1 + 2 a_{x}) \frac{1 - a_{x}}{1 + a_{x}} \approx (1 - 2 a_{x})$

The resulting errors can be neglected if | a_x | < 0.1.

2 Definitions

2.1 Specification Values of MSP430 MCU

The numeric values for the worst-case design equations are taken from MSP430F43x, MSP430F43x1, MSP430F44x Mixed-Signal Microcontrollers. The indicated values are for DV_CC = 3 V.

Table 1. Specification Values for MSP430 MCUs

Symbol	Parameter	Value
DV_CC(min)	Minimum digital supply voltage	1.8 V
DV_CC(max)	Maximum digital supply voltage	3.6 V
V_IT(max)	Maximum high input threshold voltage of a port	1.9 V
V_IT(min)	Minimum low input threshold voltage of a port	0.9 V
V_OH(min)	Minimum high port output voltage at I_O = –1.5 mA	DV_CC – 0.25 V
V_OL(max)	Maximum low port output voltage at I_O = 1.5 mA	DV_SS + 0.25 V
I_lkg	Leakage current of an input	±50 nA
	Absolute maximum current through the protection diodes of any terminal (V_I < – 0.3 V or V_I > V_CC + 0.3 V)	±2 mA

NOTE

The output impedance r_DS(on) of an MSP430 MCU output is not taken into account, due to the choice of high resistor values with the design equations. The output impedance r_DS(on)(maximum of 167 Ω) is very small compared to the resistors used.