Browse Prior Art Database

POWER FAIL RESET FOR HC11

IP.com Disclosure Number: IPCOM000005942D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2001-Nov-19
Document File: 1 page(s) / 53K

Publishing Venue

Motorola

Related People

Yona Newman: AUTHOR

Abstract

In very low cost applications with a single chip HCll, the MC34064 low-voltage detector for reset can be replaced by a simply zener diode. As shown in Figure 1, the zener diode VRl is connected from the 12V input supply to one of the micro's input capture pins (PA@ 1 or 2).

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MOlWROLA Technical Developments Volume 11 October 1990

POWER FAIL RESET FOR HCll

by Yona Newman

   In very low cost applications with a single chip HCll, the MC34064 low-voltage detector for reset can be replaced by a simply zener diode. As shown in Figure 1, the zener diode VRl is connected from the 12V input supply to one of the micro's input capture pins (PA@ 1 or 2).

   Rl maintains the zener diode in its working range which is 6.46V to 7.14V using a 6.6V 5% zener. The minimum voltage on RI for an 1lV supply will be 3.66V (11 - 7.14). This is above the minimum input high level (0.7 x Vdd) for the HCll, so the input capture pin is "1". When the supply voltage drops below Z46V (6.46 + 0.2 X Vdd), the input pin will be "0" and an input capture interrupt can be generated.

   The interrupt routine outputs via diode CR2 a logic high signal which is the power fail signal (PF). This signal switches on fet Ql so pulling down the reset pin of the micro. As the input supply continues to fall, PF will also start to fall but diode CR2 will then be reverse biased, and capacitor Cl will maintain the reset low. Resistor R3 and capacitor Cl deter- mine the length of the reset.

Resistor R3 discharges Cl so that when power returns the fet is off and a normal power-on reset can be applied. Diode CR1 and resistor R2 protect the micro input against over-voltage.

   If an analog input port is available, then Figure 2 shows another method which allows the low supply voltage limit to be set...