General_digital_IC_troubleshooting_tips

Published:2009/7/23 21:21:00 Author:Jessie | From:SeekIC

The following troubleshooting tips apply to digital circuits where the majority of components are contained in ICs. Power and ground connectionsThe first step in tracing problems in a digital circuit with ICs is to check all power and ground connections to the ICs. Many ICs have more than one power and one ground connection. For example, the LTC1043 in Fig. 1-B requires +5 V at pins 4 and 5, and -5 V at pin 17. Also, the LTC1090 in Fig. 1-C has both a digital ground (DGND) and an analog ground (AGND). Likewise, the DAC-8565 in Fig. 1-D has both an analog common (pin 5) and a digital common (pin 12). With all power and ground connections confirmed, check that all the ICs receive reset, chip-select, start, and any other function signals, as required. For example, the DAC-4881 in Fig. 1-E requires a chip-select signal at pin 1, as well as address-decode signals at pins 2 and 28. Likewise, the ADC0808/080 in Fig. 1-F requires start, ALE, EOC, and output-enable signals from the microprocessor or control logic. If any of these signals are absent or abnormal (incorrect amplitude, improper timing, etc.) circuit operation comes to an immediate halt.In some cases, control signals to digital ICs are pulses (usually timed in a certain sequence) and other control signals are steady (high or low). If any of the lines carrying the signals to the ICs are open, shorted to ground, or to power (typically +5 V or +12 V, and 3 V or 3.3 V for newer digital circuits), the IC will not function. So, if you find an IC control pin that is always high, always low, or apparently is connected to nothing (floating), check the PC traces or other wiring to that pin carefully.As an example, when the DAC4881 is connected as a 12-bit straight-binary digital-to-analog converter (Chapter 6) as shown in Fig. 1-G, the chip-select (pin 1) and address-decode (pins 2, 28) are connected to ground. If the DAC4881 is connected as an 8-bit with complementary input DAC (Fig. 1-E), the chip-select must receive a write (WR) signal, and the address-decode pins must receive address bits from the microprocessor.



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