Power-Up Considerations

2 ? 1993 Microchip Technology Inc. DS00522C-page 1 Power-Up Considerations 2 AN522 INTRODUCTION When powering up all microcontrollers it is necessary for the power supply voltage to traverse voltage ranges where the device is not guaranteed to operate before the power supply voltage reaches its final state. Since some circuits on the device (logic) will start operating at voltage levels lower than other circuits on the chip (memory), the device may power-up in an unknown state. To guarantee that the device starts up in a known state, it is necessary that it contain a power-up reset circuit. PIC16C5X microcontrollers are equipped with on-chip power-on reset circuitry, which eliminates the need for external reset logic. This circuit will function in most power-up situations where VCC rise time is fast enough (50 ms or less). This application note describes the typical power-up sequence for PIC16C5X microcon- trollers. Methods of assuring reset on power-up and after a brownout are discussed and simple, low cost external solutions are discussed for power-up situations where the PIC16C5X's internal circuitry cannot provide the reset. POWER-UP SEQUENCE The PIC16C5X incorporates complex power-on reset (POR) circuitry on-chip which provides solid, reliable internal chip reset for most power-up situations. To use this feature, the user merely needs to tie MCLR to VDD. A simplified block diagram of the on-chip reset circuitry is shown in Figure 1. On power-up, the reset latch and the start-up timer are reset to appropriate states by the power-on reset (POR). The start-up timer will begin counting once it detects MCLR to be high (i.e., external chip reset goes inactive). After the time-out period, which is typically 18 ms long, the timer will reset the reset latch and thus end the on-chip reset signal. Figures 2 and 3 are two power-up situations with relative fast rise time on VDD. In Figure 1, VDD is stable when MCLR is brought high (i.e., reset pulse is being provided by external source). The chip actually comes out reset about tOST ms after that, where tOST = oscillator start-up timer. (The timer is called oscillator start-up timer becausethetime-outwasincorporatedprimarilytoallow the crystal oscillator to stabilize on power-up.) In Figure 3, the MCLR and VDD are tied together and clearly the on-chip rest mechanism is being utilized. The VDD is stable before the start-up timer expires and there is no problem with proper reset. Figure 4, where VDD rise time is much greater than tOST (typically 18 ms) clearly is the potentially problematic situation. The POR (power-on reset) pulse comes when VDD isabout1.5V. MostCMOSlogic,includingthestart- up timer starts functioning between 1.5V to 2.0V. When the start-up timer starts times out, the chip reset is ended and the chip attempts to execute. If by this time the VDD has reached VDDMIN value, then all circuits are guaran- teed to function correctly and power-up reset is success- ful. If, however, the VDD slope was too slow and had not reached VDDMIN, then the chip may or may not function properly. FIGURE 1 - PIC16C5X INTERNAL RESET CIRCUIT Power-Up Considerations 2-11 RC OSC POWER-UP DETECT 8-BIT ASYNCH RIPPLE COUNTER (START-UP TIMER) CLEAR WDT RESET S R Q Q CHIP_RESET VDD POR (POWER-ON RESET) MCLR PIN DS00522C-page 2 ? 1993 Microchip Technology Inc. Power-Up Considerations FIGURE 2 - EXTERNAL RESET PULSE FIGURE 3 - INTERNAL RESET (VDD AND MCLR TIED TOGETHER) FIGURE 4 - INTERNAL RESET (VDD AND MCLR TIED TOGETHER): SLOW VDD RISE TIME VDD INTERNAL POR MCLR OST TIME-OUT INTERNAL RESET 0 V V1 5 V When VDD rises slowly, the internal time-out period expires long before VDD has reached its final value. In this example, the chip will reset properly if, and only if, V1 VDDMIN. tOST VDD INTERNAL POR MCLR OST TIME-OUT INTERNAL RESET tOST VDD INTERNAL POR MCLR OST TIME-OUT INTERNAL RESET tOST 2-12 2 ? 1993 Microchip Technology Inc. DS00522C-page 3 Power-Up Considerations 2 EXTERNAL POWER-ON RESET CIRCUIT To use power supplies with slow rise times it is neces- sarytouseanexternalpower-onresetcircuitsuchasthe one shown in Figure 5. This circuit uses an external RC to generate the reset pulse. The time constant of the RC should be long enough to guarantee that the reset pulse is still present until VDD has reached VDD min. R should be 40K or less to guarantee that the MCLR will pull to within 0.2 volts of VDD. (since the leakage spec on MCLR is ?5 ?A, a resistor larger than 40K may cause input high voltage on this pin to be less than VDD ? 0.2V, the required spec). The diode D is used to rapidly discharge the capacitor on power-down. This is very important as a power-up reset pulse is needed after a short power-down (less than the time constant of RC) or after a power spike. The resistor R1 protects against high current flowing into MCLR pin from fully charged capacitorCintheeventMCLRpinbreakdown is induced through ESD or EOS. The circuit, however, does not protect against brown-out situations where the power does not drop to zero, but merely dips below VDD MIN. In such a situation, voltage at the MCLR pin will not go low enough (i.e., below VIL) to guarantee a reset pulse. The following section presents an example circuit to protect against such brown-outs. FIGURE 5 - EXTERNAL POWER-ON RESET CIRCUIT BROWNOUT PROTECTION In many applications it is necessary to guarantee a reset pulse whenever VDD is less than VDD min. This can be accomplished using a brownout protection circuit such as the one shown in Figure 6. This is a simple circuit that causes a reset pulse whenever VDD drops below the zener diode voltage plus the Vbe of Q1. A 3.3 volt zener will produce a reset pulse whenever VDD drops below about 4 volts. This circuit has a typical accuracy of about ?100 mV. A less expensive, albeit less precise, brown- out circuit is shown in Figure 7. Transistor Q1 turns off when Vbe = VDD? R1/(R1+R2) falls below 0.7 V allowing R3 to pull down MCLR input. FIGURE 6 - BROWNOUT PROTECTION CIRCUIT IN4684 VDD Q1 MCLR PIC16C5X 100K 10K VDD 33K FIGURE 7 - BROWNOUT PROTECTION CIRCUIT VDD Q1 MCLR PIC16C5X R3 R3 DS00522C-page 4 ? 1993 Microchip Technology Inc. Power-Up Considerations NOTES: 2-14 WORLDWIDE SALES & SERVICE AMERICAS (continued) San Jose Microchip Technology Inc. 2107 North First Street, Suite 590 San Jose, CA 95131 Tel: 408 436-7950 Fax: 408 436-7955 ASIA/PACIFIC Hong Kong Microchip Technology Unit No. 3002-3004, Tower 1 Metroplaza 223 Hing Fong Road Kwai Fong, N.T. 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