SLAA148A October   2002  – October 2018

 

  1.   Interfacing the 3-V MSP430™ MCUs to 5-V Circuits
    1.     Trademarks
    2. 1 Introduction
    3. 2 Definitions
      1. 2.1 Specification Values of MSP430 MCU
      2. 2.2 External System Definitions
    4. 3 Input Interfaces
      1. 3.1 Resistor-Divider Input Interfaces
      2. 3.2 Transistor Input Interface
      3. 3.3 Op-Amp Input Interface
      4. 3.4 TPL7407LA Input Interface
      5. 3.5 Integrated-Circuit Input Interface
      6. 3.6 Analog Input Interface
    5. 4 Output Interfaces
      1. 4.1 Transistor Output Interface
      2. 4.2 Interface to CMOS-TTL Inputs
      3. 4.3 Interface to TPL7407LA Inputs
      4. 4.4 Op-Amp Output Interface
      5. 4.5 Integrated-Circuit Output Interface
    6. 5 Bidirectional Interfaces
      1. 5.1 Simple Bidirectional Op-Amp Interface
      2. 5.2 Integrated-Circuit I/O Interface
    7. 6 Power Supplies
    8. 7 Summary
    9. 8 References
  2.   Revision History

3.2 Transistor Input Interface

The transistor-input interface is a very simple interface that can adapt many external systems to the MSP430 family. Figure 3 shows an example for an inverting input buffer. The resistor RC can be switched off by an output to save current during low-power mode 3.

transistor-input-interface-from-a-5-v-environment.gifFigure 3. Transistor Input Interface From a 5-V Environment

The design equations for the resistors RC, RB1, and RB2 follow.

Equation 8 ensures high potential at the MSP430 with leakage currents.

Equation 8. R C   <   DV CC ( min )   -   V IT ( max ) (1   +   p )   ×   ( I lkg   +   I lkg ( Tr ) )

Equation 9 ensures turn off of the transistor for input voltage V(sysL)max.

Equation 9. R B1 R B2   >   V ( sysL ) max V BE ( off )   -  1   ×   (1   +  2 p )

Equation 10 ensures the turn on of the transistor for the input voltage V(sysH)min.

Equation 10. R B1   <   V ( sysH ) min   -   V BE ( on )   ×   1   +   R B1 R B2   ×   (1   +  2 p ) DV CC ( max )   ×   βmin   × R C min
  • VBE(off) = Transistor base-emitter voltage for secure turnoff (V)
  • VBE(on) = Transistor base-emitter voltage for secure turnon (V)
  • β = Current amplification of the transistor
  • Ilkg(Tr) = Leakage current of the transistor (A)

Example: Input voltage VI(sys) is connected to an input of the MSP430 MCU with Ilkg = ±50 nA. The minimum high-input level V(sysH)min = 4.5 V, the maximum low-input level V(sysL)max = 0.7 V. The resistor tolerance of all resistors is p = ±5%. The supply voltage is DVCC = 3 V ±10%. The transistor properties are VBE(on) = 0.75 V, VBE(off) = 0.2 V, βmin = 100, Ilkg(Tr) = 10 nA.

Equation 11 shows the maximum nominal value for RC.

Equation 11. R C   <   DV CC ( min )   -   V IT ( max ) (1   +   p )   ×   ( I lkg   +   I lkg ( Tr ) )   =   2 .7   V   -  1 .9   V (1   +  0 .05 )   ×   (50   nA   +  10   nA )   =  12 .7   MΩ R C   =  2   MΩ   is   chosen

Equation 12 shows the minimum ratio for the nominal values of RB1 and RB2.

Equation 12. R B1 R B2   >   V ( sysL ) max V BE ( off )   -  1   ×   (1   +  2 p )   =   0 .7   V 0 .2   V   -  1   ×   (1   +  0 .1 )   =  2 .75

Equation 13 shows the maximum nominal value for RB1.

Equation 13. R B1   <   V ( sysH ) min   -   V BE ( on )   ×   1   +   R B1 R B2   ×   (1   +  2 p ) DV CC ( max )   ×   βmin   × R C min R B1   <   4 .5   V   -  0 .75   V   ×   1   +  2 .75   ×   (1   +  0 .1 ) 3 .3   V   ×  100   ×2   MΩ   ×   (1   -  0 .05 )   =  85 .3 M Ω R B1   =  39   MΩ   is   chosen  

With the value 39 MΩ for RB1, Equation 14 shows the resistor RB2.

Equation 14. R B2   <   R B1 2 .75   =   39   MΩ 2 .75   =  14 .18 MΩ R B2   =  10   MΩ   is   chosen