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AD7707 Datasheet(PDF) 37 Page - Analog Devices |
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AD7707 Datasheet(HTML) 37 Page - Analog Devices |
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37 / 53 page  AD7707 Rev. B | Page 36 of 52 DIGITAL INTERFACE As previously outlined, the AD7707’s programmable functions are controlled using a set of on-chip registers. Data is written to these registers via the part’s serial interface and read access to the on-chip registers is also provided by this interface. All commu- nications to the part must start with a write operation to the communications register. After power-on or RESET, the device expects a write to its communications register. The data written to this register determines whether the next operation to the part is a read or a write operation and also determines to which register this read or write operation occurs. Therefore, write access to any of the other registers on the part starts with a write operation to the communications register followed by a write to the selected register. A read operation from any other register on the part (including the data register) starts with a write operation to the communications register followed by a read operation from the selected register. The AD7707 serial interface consists of five signals CS, SCLK, DIN, DOUT, and DRDY. The DIN line is used for transferring data into the on-chip registers, and 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) take place with respect to this SCLK signal. The DRDY line is used as a status signal to indicate when data is ready to be read from the AD7707 data register. DRDY goes low when a new data word is available in the output 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 output register to indicate when not to read from the device to ensure that a data read is not attempted while the register is being updated. CS is used to select the device. It can be used to decode the AD7707 in systems where a number of parts are connected to the serial bus. Figure 20 and Figure 21 show timing diagrams for interfacing to the AD7707 with CS used to decode the part. is for a read operation from the AD7707’s output shift register whereas shows a write operation to the input shift register. It is possible to read the same data twice from the output register even though the Figure 20 Figure 21 DRDY line returns high after the first read operation. Care must be taken, however, to ensure that the read operations have been completed before the next output update is about to take place. The AD7707 serial interface can operate in 3-wire mode by tying the CS input low. In this case, the SCLK, DIN, and DOUT lines are used to communicate with the AD7707 and the status of DRDY can be obtained by interrogating the MSB of the communications register. This scheme is suitable for interfacing to microcontrollers. If CS is required as a decoding signal, it can be generated from a port bit. For microcontroller interfaces, it is recommended that SCLK idles high between data transfers. The AD7707 can also be operated with CS used as a frame synchronization signal. This scheme is suitable for DSP interfaces. In this case, the first bit (MSB) is effectively clocked out by CS because CS normally occurs after the falling edge of SCLK in DSPs. The SCLK can continue to run between data transfers provided the timing numbers are obeyed. The serial interface can be reset by exercising the RESET input on the part. It can also be reset by writing a series of 1s on the DIN input. If a Logic 1 is written to the AD7707 DIN line for at least 32 serial clock cycles, the serial interface is reset. This ensures that in 3-wire systems, if the interface is lost either via a software error or by a glitch in the system, it can be reset back to a known state. This state returns the interface to where the AD7707 is expecting a write operation to its communications register. This operation in itself does not reset the contents of any registers but because the interface was lost, the information written to any of the registers is unknown and it is advisable to set up all registers again. Some microprocessor or microcontroller serial interfaces have a single serial data line. In this case, it is possible to connect the AD7707’s DATA OUT and DATA IN lines together and connect them to the single data line of the processor. A 10 kΩ pull-up resistor should be used on this single data line. In this case, if the interface is lost, because the read and write operations share the same line, the procedure to reset it back to a known state is somewhat different than previously described. It requires a read operation of 24 serial clocks followed by a write operation where a Logic 1 is written for at least 32 serial clock cycles to ensure that the serial interface is back into a known state. |
Similar Part No. - AD7707_17 |
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Similar Description - AD7707_17 |
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