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PIC16F84A Datasheet(PDF) 46 Page - Microchip Technology |
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PIC16F84A Datasheet(HTML) 46 Page - Microchip Technology |
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46 / 124 page  PIC16F8X DS30430C-page 46 © 1998 Microchip Technology Inc. 8.7 Time-out Sequence and Power-down Status Bits (TO/PD) On power-up (Figure 8-10, Figure 8-11, Figure 8-12 and Figure 8-13) the time-out sequence is as follows: First PWRT time-out is invoked after a POR has expired. Then the OST is activated. The total time-out will vary based on oscillator configuration and PWRTE configuration bit status. For example, in RC mode with the PWRT disabled, there will be no time-out at all. TABLE 8-5 TIME-OUT IN VARIOUS SITUATIONS Since the time-outs occur from the POR reset pulse, if MCLR is kept low long enough, the time-outs will expire. Then bringing MCLR high, execution will begin immediately (Figure 8-10). This is useful for testing purposes or to synchronize more than one PIC16F8X device when operating in parallel. Table 8-6 shows the significance of the TO and PD bits. Table 8-3 lists the reset conditions for some special registers, while Table 8-4 lists the reset conditions for all the registers. TABLE 8-6 STATUS BITS AND THEIR SIGNIFICANCE 8.8 Reset on Brown-Out A brown-out is a condition where device power (VDD) dips below its minimum value, but not to zero, and then recovers. The device should be reset in the event of a brown-out. To reset a PIC16F8X device when a brown-out occurs, external brown-out protection circuits may be built, as shown in Figure 8-14 and Figure 8-15. FIGURE 8-14: BROWN-OUT PROTECTION CIRCUIT 1 FIGURE 8-15: BROWN-OUT PROTECTION CIRCUIT 2 Oscillator Configuration Power-up Wake-up from SLEEP PWRT Enabled PWRT Disabled XT, HS, LP 72 ms + 1024TOSC 1024TOSC 1024TOSC RC 72 ms — — TO PD Condition 1 1 Power-on Reset 0 x Illegal, TO is set on POR x 0 Illegal, PD is set on POR 0 1 WDT Reset (during normal operation) 0 0 WDT Wake-up 1 1 MCLR Reset during normal operation 1 0 MCLR Reset during SLEEP or interrupt wake-up from SLEEP This circuit will activate reset when VDD goes below (Vz + 0.7V) where Vz = Zener voltage. VDD 33k 10k 40k VDD MCLR PIC16F8X This brown-out circuit is less expensive, although less accurate. Transistor Q1 turns off when VDD is below a certain level such that: VDD • R1 R1 + R2 = 0.7V R2 40k VDD MCLR PIC16F8X R1 Q1 VDD |
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