SLAA148A October   2002  – October 2018 MSP430BT5190 , MSP430F1101 , MSP430F1101A , MSP430F1111A , MSP430F112 , MSP430F1121 , MSP430F1121A , MSP430F1122 , MSP430F1132 , MSP430F122 , MSP430F1222 , MSP430F123 , MSP430F1232 , MSP430F133 , MSP430F135 , MSP430F147 , MSP430F1471 , MSP430F148 , MSP430F1481 , MSP430F149 , MSP430F1491 , MSP430F155 , MSP430F156 , MSP430F157 , MSP430F1610 , MSP430F1611 , MSP430F1612 , MSP430F167 , MSP430F168 , MSP430F169 , MSP430F2001 , MSP430F2002 , MSP430F2003 , MSP430F2011 , MSP430F2012 , MSP430F2013 , MSP430F2013-EP , MSP430F2101 , MSP430F2111 , MSP430F2112 , MSP430F2121 , MSP430F2122 , MSP430F2131 , MSP430F2132 , MSP430F2232 , MSP430F2234 , MSP430F2252 , MSP430F2252-Q1 , MSP430F2254 , MSP430F2272 , MSP430F2272-Q1 , MSP430F233 , MSP430F2330 , MSP430F235 , MSP430F2350 , MSP430F2370 , MSP430F2410 , MSP430F2416 , MSP430F2417 , MSP430F2418 , MSP430F2419 , MSP430F247 , MSP430F2471 , MSP430F248 , MSP430F2481 , MSP430F249 , MSP430F2491 , MSP430F2616 , MSP430F2617 , MSP430F2618 , MSP430F2619 , MSP430F412 , MSP430F413 , MSP430F4132 , MSP430F415 , MSP430F4152 , MSP430F417 , MSP430F423 , MSP430F423A , MSP430F425 , MSP430F4250 , MSP430F425A , MSP430F4260 , MSP430F427 , MSP430F4270 , MSP430F427A , MSP430F435 , MSP430F4351 , MSP430F436 , MSP430F4361 , MSP430F437 , MSP430F4371 , MSP430F438 , MSP430F439 , MSP430F447 , MSP430F448 , MSP430F4481 , MSP430F449 , MSP430F4491 , MSP430F4616 , MSP430F46161 , MSP430F4617 , MSP430F46171 , MSP430F4618 , MSP430F46181 , MSP430F4619 , MSP430F46191 , MSP430F47126 , MSP430F47127 , MSP430F47163 , MSP430F47166 , MSP430F47167 , MSP430F47173 , MSP430F47176 , MSP430F47177 , MSP430F47183 , MSP430F47186 , MSP430F47187 , MSP430F47193 , MSP430F47196 , MSP430F47197 , MSP430F477 , MSP430F478 , MSP430F4783 , MSP430F4784 , MSP430F479 , MSP430F4793 , MSP430F4794 , MSP430F5232 , MSP430F5234 , MSP430F5237 , MSP430F5239 , MSP430F5242 , MSP430F5244 , MSP430F5247 , MSP430F5249 , MSP430F5252 , MSP430F5253 , MSP430F5254 , MSP430F5255 , MSP430F5256 , MSP430F5257 , MSP430F5258 , MSP430F5259 , MSP430F5304 , MSP430F5308 , MSP430F5309 , MSP430F5310 , MSP430F5324 , MSP430F5325 , MSP430F5326 , MSP430F5327 , MSP430F5328 , MSP430F5329 , MSP430F5333 , MSP430F5336 , MSP430F5338 , MSP430F5340 , MSP430F5341 , MSP430F5342 , MSP430F5418 , MSP430F5418A , MSP430F5419 , MSP430F5419A , MSP430F5435 , MSP430F5435A , MSP430F5436 , MSP430F5436A , MSP430F5437 , MSP430F5437A , MSP430F5438 , MSP430F5500 , MSP430F5501 , MSP430F5502 , MSP430F5503 , MSP430F5504 , MSP430F5505 , MSP430F5506 , MSP430F5507 , MSP430F5508 , MSP430F5509 , MSP430F5510 , MSP430F5630 , MSP430F5631 , MSP430F5632 , MSP430F5633 , MSP430F5634 , MSP430F5635 , MSP430F5636 , MSP430F5637 , MSP430F5638 , MSP430F6433 , MSP430F6435 , MSP430F6436 , MSP430F6438 , MSP430F6630 , MSP430F6631 , MSP430F6632 , MSP430F6633 , MSP430F6634 , MSP430F6635 , MSP430F6636 , MSP430F6637 , MSP430F6638 , MSP430FE423 , MSP430FE4232 , MSP430FE423A , MSP430FE4242 , MSP430FE425 , MSP430FE4252 , MSP430FE425A , MSP430FE427 , MSP430FE4272 , MSP430FE427A , MSP430FG4250 , MSP430FG4260 , MSP430FG4270 , MSP430FG4616 , MSP430FG4617 , MSP430FG4618 , MSP430FG4619 , MSP430FG477 , MSP430FG478 , MSP430FG479 , MSP430FW423 , MSP430FW425 , MSP430FW427 , MSP430FW428 , MSP430FW429 , MSP430G2001 , MSP430G2101 , MSP430G2102 , MSP430G2111 , MSP430G2112 , MSP430G2121 , MSP430G2131 , MSP430G2132 , MSP430G2152 , MSP430G2201 , MSP430G2201-Q1 , MSP430G2211 , MSP430G2212 , MSP430G2221 , MSP430G2231-Q1 , MSP430G2232 , MSP430G2252 , MSP430G2302 , MSP430G2312 , MSP430G2332 , MSP430G2352 , MSP430G2402 , MSP430G2432 , MSP430G2452 , MSP430L092

 

  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