How does the serial interface of the AD719x operate?
The serial interface on this family of converters is implemented as a state machine. The interface works by counting clocks on each data transfer. Therefore, if you are performing a write operation to one of the ADC’s 16-bit registers, 16 SCLK cycles must be applied to the ADC and, the converter will transfer in the 16 bits of data on the DIN line on each of the sixteen SCLK cycles. When this operation is complete, the part returns to a state in which it expects the next operation to be a write operation to the communications register. This happens regardless of which register has been accessed and whether a read or write operation has taken place. The ADC knows how many clock cycles should be in a transfer to/from a particular register and in this way knows when the transfer is complete.
The first bit of the communications register is a gating bit that must be set to 0 to access the communications register. If there is a 1 on the DIN line when the part is expecting a write to this bit, the part effectively wraps itself around to monitoring that bit. If there is a 0 on the DIN line when the part is expecting a write to this bit, the part continues to load the next 7 bits of the communications register on the next 7 SCLK cycles.
The serial interface on these converters consists of four signals: , SCLK, DIN, DOUT/. The DIN line is used for transferring data into the on-chip registers while the DOUT/ line is used for accessing data from the on-chip registers. SCLK is the serial clock input for the device and all data transfers (either on DIN or DOUT/) occur with respect to this SCLK signal. The DOUT/ pin operates as a data ready signal also, the line going low when a new conversion is available in the data register. It is reset high when a read operation from the data register is complete. It also goes high prior to the updating of the data register to indicate when not to read from the device. This is to ensure that a data read is not attempted while the register is being updated. is used to select the device. It can be used to decode the individual converters in systems where a number of parts are connected to the serial bus.
The serial interface can operate in three-wire mode by tying the input low. In this case, the SCLK, DIN and DOUT/ lines are used to communicate with the converter. This scheme is suitable for interfacing to microcontrollers. If is required as a decoding signal, it can be generated from a port bit. For microcontroller interfaces, it is recommended that the SCLK idle high between data transfers.
The ADC registers are either 8 or 24 bits, there are no 16 bit registers.
you are correct - the registers on the AD719x family are either 8-bit or 24-bit. Therefore, if writing to a 24-bit reigster, 24 SCLK pulses are needed. When writing to an 8-bit register, 8 SCLK pulses are required. The important point is that the correct number of SCLK pulses are applied. Too many SCLK pulses or too little SCLK pulses will cause the serial interface to go asynchronous.
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