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AD7674 Datasheet(PDF) 24 Page - Analog Devices |
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AD7674 Datasheet(HTML) 24 Page - Analog Devices |
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24 / 28 page 
AD7674 Rev. A | Page 24 of 28 SDOUT SCLK D1 D0 X D17 D16 D15 12 3 16 17 18 BUSY INVSCLK = 0 EXT/INT = 1 RD = 0 t35 t36 t37 t31 t32 t16 t3 CS CNVST 03083-0-043 Figure 43. Slave Serial Data Timing for Reading (Read Previous Conversion during Convert) BUSY BUSY AD7674 #2 (UPSTREAM) AD7674 #1 (DOWNSTREAM) RDC/SDIN SDOUT CNVST CS SCLK RDC/SDIN SDOUT CNVST CS SCLK DATA OUT SCLK IN CS IN CNVST IN BUSY OUT 03083-0-044 Figure 44. Two AD7674s in a Daisy-Chain Configuration External Clock Data Read during Conversion Figure 43 shows the detailed timing diagrams of this method. During a conversion, while both CS and RD are low, the result of the previous conversion can be read. The data is shifted out MSB first with 18 clock pulses, and is valid on both the rising and falling edge of the clock. The 18 bits have to be read before the current conversion is complete. If that is not done, RDERROR is pulsed high and can be used to interrupt the host interface to prevent incomplete data reading. There is no daisy- chain feature in this mode, and the RDC/SDIN input should always be tied either high or low. To reduce performance degradation due to digital activity, a fast discontinuous clock is recommended to ensure that all bits are read during the first half of the conversion phase. It is also possible to begin to read the data after conversion and continue to read the last bits even after a new conversion has been initiated. MICROPROCESSOR INTERFACING The AD7674 is ideally suited for traditional dc measurement applications supporting a microprocessor, and for ac signal processing applications interfacing to a digital signal processor. The AD7674 is designed to interface either with a parallel 8-bit or 16-bit wide interface, or with a general-purpose serial port or I/O ports on a microcontroller. A variety of external buffers can be used with the AD7674 to prevent digital noise from coupling into the ADC. The following section illustrates the use of the AD7674 with an SPI equipped DSP, the ADSP-219x. SPI Interface (ADSP-219x) Figure 45 shows an interface diagram between the AD7674 and the SPI equipped ADSP-219x. To accommodate the slower speed of the DSP, the AD7674 acts as a slave device, and data must be read after conversion. This mode also allows the daisy- chain feature. The convert command could be initiated in response to an internal timer interrupt. The 18-bit output data are read with 3-byte SPI access. The reading process could be initiated in response to the end-of-conversion signal (BUSY going low) using an interrupt line of the DSP. The serial interface (SPI) on the ADSP-219x is configured for master mode (MSTR) = 1, Clock Polarity Bit (CPOL) = 0, Clock Phase Bit (CPHA) = 1, and SPI interrupt enable (TIMOD) = 00, by writing to the SPI Control register (SPICLTx). It should be noted that to meet all timing requirements, the SPI clock should be limited to 17 Mbps, which allows it to read an ADC result in about 1.1 μs. When a higher sampling rate is desired, use of one of the parallel interface modes is recommended. AD76741 ADSP-219x1 PFx MISOx SCKx PFx OR TFSx SDOUT SCLK CNVST CS RD INVSCLK 1ADDITIONAL PINS OMITTED FOR CLARITY. SPIxSEL (PFx) BUSY DVDD MODE0/MODE1 EXT/INT Figure 45. Interfacing the AD7674 to an SPI Interface |
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