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AD7707 Datasheet(PDF) 40 Page - Analog Devices |
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AD7707 Datasheet(HTML) 40 Page - Analog Devices |
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40 / 53 page  AD7707 Rev. B | Page 39 of 52 MICROCOMPUTER/MICROPROCESSOR INTERFACING The AD7707’s flexible serial interface allows for easy interface to most microcomputers and microprocessors. The flowchart of Figure 22 outlines the sequence that should be followed when interfacing a microcontroller or microprocessor to the AD7707. Figure 23 and Figure 24 show some typical interface circuits. The serial interface on the AD7707 is capable of operating from just three wires and is compatible with SPI interface protocols. The 3-wire operation makes the part ideal for isolated systems in which minimizing the number of interface lines also minimizes the number of opto-isolators required in the system. The serial clock input is a Schmitt-triggered input to accommodate slow edges from optocouplers. The rise and fall times of other digital inputs to the AD7707 should be no longer than 1 μs. Most of the registers on the AD7707 are 8-bit registers, which facilitates easy interfacing to the 8-bit serial ports of microcon- trollers. The data register on the AD7707 is 16 bits, and the zero-scale and full-scale calibration registers are 24-bit registers but data transfers to these registers can consist of multiple 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 AD7707. Even though some of the registers on the AD7707 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 (indicating that the next operation is a write to the setup register) and then write eight bits to the setup register. If required, this can all be performed in a single 16-bit transfer 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. AD7707 TO 68HC11 INTERFACE Figure 23 shows an interface between the AD7707 and the 68HC11 microcontroller. The diagram shows the minimum (3-wire) interface with CS on the AD7707 hard-wired 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 AD7707. The monitoring of the DRDY line can be done in two ways. First, DRDY can be connected to one of the 68HC11 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 AD7707, 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. SS SCK MISO MOSI 68HC11 RESET SCLK DOUT DIN CS AD7707 VDD VDD Figure 23. AD7707-to-68HC11 Interface The 68HC11 is configured in 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 AD7707 is not capable of full duplex operation. If the AD7707 is configured for a write operation, no data appears on the DATA OUT lines even when the SCLK input is active. Similarly, if the AD7707 is configured for a read operation, data presented to the part on the DATA IN line is ignored even when SCLK is active. Coding for an interface between the 68HC11 and the AD7707 is given in the C Code for Interfacing AD7707 to 68HC11 section. In this example, the DRDY output line of the AD7707 is connected to the PC0 port bit of the 68HC11 and is polled to determine its status. |
Similar Part No. - AD7707_17 |
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Similar Description - AD7707_17 |
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