When information (conversion data or information from the on-chip registers) is read from the AD779x ADC, the LSB readback is always 1. Why ?

Question

ADIApproved · Accepted Answer

The DOUT and RDY functions share a pin on the AD779x. Every time a conversion is completed, the pin goes low, indicating to the microprocessor that a valid conversion is available. When the user requests a read of the data register, the DOUT/RDY pin functions as a DOUT pin. When pulses are applied to the SCLK pin, the data is placed on the DOUT pin. The data is output from the AD779x following the SCLK falling edge and is valid on the SCLK rising edge. When the LSB of the data is output, the DOUT/RDY pin functions as a RDY pin again. The change from the DOUT to the RDY function occurs on the last SCLK rising edge. The microprocessor is latching the bits on the SCLK rising edge. So, if the microprocessor is slow, then the DOUT/RDY pin is functioning as a RDY pin when the LSB is latched into the microprocessor. So, the microprocessor reads the value of the RDY pin rather than the LSB, causing the LSB to be a 1. To prevent this, a faster microprocessor must be used. Alternatively, general-purpose input/output pins of the microprocessor can be used to represent a serial interface. By bit-banging, the user has more control over the read instant. By reading the values on the DOUT pin when SCLK is low rather than latching in the data on the SCLK rising edge, all bits of the data read will be valid.)

EZAdministrator · Answer

This question has been closed by the EZ team and is assumed answered.

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When information (conversion data or information from the on-chip registers) is read from the AD779x ADC, the LSB readback is always 1. Why ?