Supply Voltage Supervisor TL77xx Series
ABSTRACT
After power-on a digital system must be forced into a definite initial state. For microcomputers and microprocessors a
Reset input is provided to which, in simple applications, an R/C network is connected. After power-on, this circuitry
maintains the logic level at this input high (or low), until the supply voltage has reached its nominal value, and the
internal logic of the microcomputer has executed the Reset. However, this simple circuit does not work well under all
conditions, or during short drops of the supply voltage. This applications report describes the operation and the
application of the supply voltage supervisor circuits series TL77xx, which has been specially designed to solve these
applications problems.
1. Introduction
After power-on a digital system must normally be forced into a definite initial state. For microcomputers
and microprocessors a reset input is provided to which, in simple applications, an R/C network is
connected. After power-on, this circuitry maintains the logic level at this input high (or low), until the supply
voltage has reached its nominal value, and the internal logic of the microcomputer has executed the
initialization of the system.
Figure 1. Reset circuit
However, this circuit does not work well during short drops of the supply voltage. In this case, the capacitor
C must be quickly discharged via the diode D, which will occur only if the supply voltage V goes below the
cc
threshold voltage (1 ¼ 2 Volt) of the Reset input. A small decrease of V under the recommended
cc
minimum supply voltage can destroy the content of the memory and registers and yet not activate the reset
circuit. This may have catastrophic consequences.
The following program example (8080 assembler code) tests an input:
WAIT
LOOP
LXI
LDAX
ANI
B,INPUT
B
MASK
LOOP
;load address
;read input
;mask bit
;test
JNZ
If, during the execution of the loop, the content of the B/C register is affected by a short voltage drop
(causing an incorrect input to be read), an incorrect condition will be tested and an incorrect decision will
be the consequence. Alternatively the addressed input will show a value which can never give a positive
test result: the execution of the program seems to be stopped.
Supply voltage Supervisor TL77xx Series
1
TI [ TEXAS INSTRUMENTS ]
