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AD7715 Datasheet(PDF) 33 Page - Analog Devices |
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AD7715 Datasheet(HTML) 33 Page - Analog Devices |
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33 / 41 page  AD7715 Data Sheet Rev. E | Page 32 of 40 MICROCONTROLLER/MICROPROCESSOR INTERFACING The flexible serial interface of the AD7715 allows for easy interface to most microcontrollers and microprocessors. The flow chart of Figure 10 outlines the sequence which should be followed when interfacing a microcontroller or microprocessor to the AD7715. Figure 11, Figure 12, and Figure 13 show some typical interface circuits. The serial interface on the AD7715 has the capability of operating from just three wires and is compatible with SPI interface protocols. The three-wire operation makes the part ideal for isolated systems where minimizing the number of interface lines minimizes the number of opto-isolators required in the system. The rise and fall times of the digital inputs to the AD7715 (especially the SCLK input) should be no longer than 1 µs. Most of the registers on the AD7715 are 8-bit registers. This facilitates easy interfacing to the 8-bit serial ports of micro- controllers. Some of the registers on the part are up to 16 bits, but data transfers to these 16-bit registers can consist of a full 16-bit transfer or two 8-bit transfers to the serial port of the microcontroller. DSP processors and microprocessors generally transfer 16 bits of data in a serial data operation. Some of these processors, such as the ADSP-2105, have the facility to program the amount of cycles in a serial transfer. This allows the user to tailor the number of bits in any transfer to match the register length of the required register in the AD7715. Even though some of the registers on the AD7715 are only eight bits in length, communicating with two of these registers in successive write operations can be handled as a single 16-bit data transfer if required. For example, if the setup register is to be updated, the processor must first write to the communications register (saying that the next operation is a write to the setup register) and then write eight bits to the setup register. This can all be done in a single 16-bit transfer if required because once the eight serial clocks of the write operation to the communications register have been completed, the part immediately sets itself up for a write operation to the setup register. AD7715 TO 68HC11 INTERFACE Figure 11 shows an interface between the AD7715 and the 68HC11 microcontroller. The diagram shows the minimum (three-wire) interface with CS on the AD7715 hardwired low. In this scheme, the DRDY bit of the communications register is monitored to determine when the data register is updated. An alternative scheme, which increases the number of interface lines to four, is to monitor the DRDY output line from the AD7715. The monitoring of the DRDY line can be done in two ways. First, DRDY can be connected to one of the 68HC11’s port bits (such as PC0) which is configured as an input. This port bit is then polled to determine the status of DRDY. The second scheme is to use an interrupt driven system, in which case the DRDY output is connected to the IRQ input of the 68HC11. For interfaces that require control of the CS input on the AD7715, one of the port bits of the 68HC11 (such as PC1), which is configured as an output, can be used to drive the CS input. AD7715 DOUT SCLK CS 68HC11 SS DVDD DVDD RESET DIN SCK MISO MOSI Figure 11. AD7715 to 68HC11 Interface The 68HC11 is configured in the master mode with its CPOL bit set to a Logic 1 and its CPHA bit set to a Logic 1. When the 68HC11 is configured like this, its SCLK line idles high between data transfers. The AD7715 is not capable of full duplex operation. If the AD7715 is configured for a write operation, no data appears on the DOUT lines even when the SCLK input is active. Similarly, if the AD7715 is configured for a read operation, data presented to the part on the DIN line is ignored even when SCLK is active. Coding for an interface between the 68HC11 and the AD7715 is given in the C Code for Interfacing AD7715 to 68HC11 section. In this example, the DRDY output line of the AD7715 is con- nected to the PC0 port bit of the 68HC11 and is polled to determine its status. |
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