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TL7705 데이터시트(PDF) 5 Page - Texas Instruments |
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TL7705 데이터시트(HTML) 5 Page - Texas Instruments |
5 / 17 page Supply voltage Supervisor TL77xx Series 1 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 cc goes below the threshold voltage (1 … 2 Volt) of the Reset input. A small decrease of V cc under the recommended 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 LXI B,INPUT ;load address LOOP LDAX B ;read input ANI MASK ;mask bit JNZ LOOP ;test 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. |
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