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AD8804AN Datasheet(PDF) 9 Page - Analog Devices |
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AD8804AN Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page AD8802/AD8804 REV. 0 –9– A VDD VREFH GND VREFL AD8802/ AD8804 B +5V +12V –5V OP191 OP193 RF 100k Ω RS 100k Ω –5V TO +4.98V 0V TO +10V 100k Ω 100k Ω +5V AD8804 ONLY Figure 23. Increasing Output Voltage Swing DAC B of Figure 24 is in a noninverting gain of two configura- tions, which increases the available output swing to +10 V. The feedback resistors can be adjusted to provide any scaling of the output voltage, within the limits of the external op amp power supplies. Microcomputer Interfaces The AD8802/AD8804 serial data input provides an easy inter- face to a variety of single-chip microcomputers ( µCs). Many µCs have a built-in serial data capability that can be used for com- municating with the DAC. In cases where no serial port is pro- vided, or it is being used for some other purpose (such as an RS-232 communications interface), the AD8802/AD8804 can easily be addressed in software. Twelve data bits are required to load a value into the AD8802/ AD8804 (4 bits for the DAC address and 8 bits for the DAC value). If more than 12 bits are transmitted before the Chip Se- lect input goes high, the extra (i.e., the most-significant) bits are ignored. This feature is valuable because most µCs only transmit data in 8-bit increments. Thus, the µC will send 16 bits to the DAC instead of 12 bits. The AD8802/AD8804 will only re- spond to the last 12 bits clocked into the SDI port, however, so the serial data interface is not affected. An 8051 µC Interface A typical interface between the AD8802/AD8804 and an 8051 µC is shown in Figure 24. This interface uses the 8051’s internal serial port. The serial port is programmed for Mode 0 opera- tion, which functions as a simple 8-bit shift register. The 8051’s Port 3.0 pin functions as the serial data output, while Port 3.1 serves as the serial clock. When data is written to the Serial Buffer Register (SBUF, at Special Function Register location 99H), the data is automati- cally converted to serial format and clocked out via Port 3.0 and Port 3.1. After 8 bits have been transmitted, the Transmit Inter- rupt flag (SCON.1) is set and the next 8 bits can be transmitted. The AD8802 and AD8804 require the Chip Select to go low at the beginning of the serial data transfer. In addition, the SCLK input must be high when the Chip Select input goes high at the end of the transfer. The 8051’s serial clock meets this require- ment, since Port 3.1 both begins and ends the serial data in the high state. +5V P3.0 P3.1 P1.3 P1.2 P1.1 SERIAL DATA SHIFT REGISTER RxD TxD SHIFT CLOCK 1.1 1.2 1.3 PORT 1 SBUF 8051 µC 0.1µF 10µF O1 O12 GND AD8802 SDI SCLK RESET SHDN CS VREFH VDD Figure 24. Interfacing the 8051 µC to an AD8802/AD8804, Using the Serial Port Software for the 8051 Interface A software for the AD8802/AD8804 to 8051 interface is shown in Listing 1. The routine transters the 8-bit data stored at data memory location DAC_VALUE to the AD8802/AD8804 DAC addressed by the contents of location DAC_ADDR. The subroutine begins by setting appropriate bits in the Serial Control register to configure the serial port for Mode 0 opera- tion. Next the DAC’s Chip Select input is set low to enable the AD8802/AD8804. The DAC address is obtained from memory location DAC_ADDR, adjusted to compensate for the 8051’s serial data format, and moved to the serial buffer register. At this point, serial data transmission begins automatically. When all 8 bits have been sent, the Transmit Interrupt bit is set, and the subroutine then proceeds to send the DAC value stored at location DAC_VALUE. Finally the Chip Select input is re- turned high, causing the appropriate AD8802/AD8804 output voltage to change, and the subroutine ends. The 8051 sends data out of its shift register LSB first, while the AD8802/AD8804 require data MSB first. The subroutine there- fore includes a BYTESWAP subroutine to reformat the data. This routine transfers the MSB-first byte at location SHIFT1 to an LSB-first byte at location SHIFT2. The routine rotates the MSB of the first byte into the carry with a Rotate Left Carry in- struction, then rotates the carry into the MSB of the second byte with a Rotate Right Carry instruction. After 8 loops, SHIFT2 contains the data in the proper format. The BYTESWAP routine in Listing 1 is convenient because the DAC data can be calculated in normal LSB form. For example, producing a ramp voltage on a DAC is simply a matter of re- peatedly incrementing the DAC_VALUE location and calling the LD_8802 subroutine. If the µC’s hardware serial port is being used for other purposes, the AD8802/AD8804 DAC can be loaded by using the parallel port. A typical parallel interface is shown in Figure 25. The se- rial data is transmitted to the DAC via the 8051’s Port 1.6 out- put, while Port 1.6 acts as the serial clock. Software for the interface of Figure 25 is contained in Listing 2. The subroutine will send the value stored at location DAC_VALUE to the AD8802/AD8804 DAC addressed by location DAC_ADDR. The program begins by setting the AD8802/AD8804’s Serial Clock and Chip Select inputs high, then setting Chip Select low |
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