The second mode is where the serial port acts as a slave device to an external controller, where the serial data is output from the port according to the clock provided by the external de- vice (this mode is the SEC mode, synchronous external clock mode). The serial port on the converter can operate in either master mode, where the converter itself outputs the data according to its own timing (this mode is called the SSC mode, synchronous self-clock mode). The serial interface provides two different modes of operation. This reduces the package pin count and also reduces the number of digital interface pins that can radiate noise onto the converter die. An- other option would be to provide a second set of latches outside the A/D to isolate the bus activity from being on the digital output pins of the converter itself.Ĭirrus Logic has chosen to offer the CS556X/7X/8X series of converters with only a serial interface. Many A/D converters that have parallel interfaces recommend that bus activity be stopped when a conversion is performed, and that the data from the converter be read when the converter is not performing a conversion. The digital activity on the bus need not be due to the converter itself but may be caused by other peripherals using the same bus. This electromagnetic field can interfere with the performance of the converter.
When the digital data on the digital bus is changing each bond wire acts as a radiator of an electromagnetic field. The data interface has many more connections via bond wires to the semiconductor die. This factor can increase the cost, but a larger concern should be how a parallel interface can reduce performance. A parallel interface requires many more pins on the A/D than a serial interface.
While this makes for faster data transfer it can introduce some possible compromises in the performance of the A/D converter. Some A/D converters provide parallel output data as an output option. The document also discussed the CS556x/7x/8x series of delta-sigma A/D converters, and how the conversion is being done using these converters. This application note presents the process of converting a serial output into parallel output using common serial-to-parallel register integrated circuits.
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