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AD7868 Datasheet(PDF) 11 Page - Analog Devices |
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AD7868 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD7868 –11– REV. B MICROPROCESSOR INTERFACING Microprocessor interfacing to the AD7868 is via a serial bus that uses standard protocol compatible with DSP machines. The communication interface consists of separate transmit (DAC) and receive (ADC) sections whose operations can be either syn- chronous or asynchronous with respect to each other. Each sec- tion has a clock signal, a data signal and a frame or strobe pulse. Synchronous operation means that data is transmitted from the ADC and to the DAC at the same time. In this mode only one interface clock is needed and this has to be the ADC clock out, so RCLK must be connected to TCLK. For asynchronous op- eration, DAC and ADC data transfers are independent of each other, the ADC provides the receive clock (RCLK) while the transmit clock (TCLK) may be provided by the processor or the ADC or some other external clock source. Another option to be considered with serial interfacing is the use of a gated clock. A gated clock means that the device that is sending the data switches on the clock when data is ready to be transmitted and three states the clock output when transmission is complete. Only 16 clock pulses are transmitted with the first data bit getting latched into the receiving device on the first fall- ing clock edge. Ideally, there is no need for frame pulses, how- ever, the AD7868 DAC frame input (TFS) has to be driven high between data transmissions. The easiest method is to use RFS to drive TFS and use only synchronous interfacing. This avoids the use of interconnects between the processor and AD7868 frame signals. Not all processors have a gated clock facility, Figure 16 shows an example with the DSP56000. Table I below shows the number of interconnect lines between the processor and the AD7868 for the different interfacing op- tions. The AD7868 has the facility to use different clocks for transmitting and receiving data. This option, however, only ex- ists on some processors and normally just one clock (ADC clock) is used for all communication with the AD7868. For sim- plicity, all the interface examples in this data sheet use synchro- nous interfacing and use the ADC clock (RCLK) as an input for the DAC clock (TCLK). For a better understanding of each of these interfaces, consult the relevant processor data sheet. Table I. Interconnect Lines for Different Interfacing Options No. of Configuration Interconnects Signals Synchronous 4 RCLK, DR, DT and RFS (TCLK = RCLK, TFS = RFS) Asynchronous* 5 or 6 RCLK, DR, RFS, DT, TFS (TCLK = RCLK or µP serial CLK) Synchronous 3 RCLK, DR and DT Gated Clock (TCLK = RCLK, TFS = RFS) *5 LINES OF INTERCONNECT WHEN TCLK = RCLK 6 LINES OF INTERCONNECT WHEN TCLK = µP SERIAL CLK AD7868—DSP56000 Interface Figure 16 shows a typical interface between the AD7868 and DSP56000. The interface arrangement is synchronous with a gated clock requiring only three lines of interconnect. The DSP56000 internal serial control registers have to be configured for a 16-bit data word with valid data on the first falling clock edge. Conversion starts and DAC updating are controlled by an external timer. Data transfers, which occur during ADC conver- sions, are between the processor receive and transmit shift regis- ters and the AD7868’s ADC and DAC. At the end of each 16-bit transfer the DSP56000 receives an internal interrupt indi- cating the transmit register is empty and the receive register is full. DSP56000 STD TFS *ADDITIONAL PINS OMITTED FOR CLARITY DT SCK SRD RCLK DR CONVST RFS TIMER AD7868* 4.7k Ω 2k Ω 4.7k Ω LDAC CONTROL TCLK 5V + SC0 Figure 16. AD7868—DSP56000 Interface AD7868—ADSP-2101/ADSP-2102 Interface An interface which is suitable for the ADSP-2101 or the ADSP- 2102 is shown in Figure 17. The interface is configured for syn- chronous, continuous clock operation. The LDAC is tied low so the DAC gets updated on the sixteenth falling clock after TFS goes low. Alternatively LDAC may be driven from a timer as shown in Figure 16. As with the previous interface the processor receives an interrupt after reading or writing to the AD7868 and updates its own internal registers in preparation for the next data transfer. ADSP-2101/ ADSP-2102 TFS DT TCLK DT LDAC TFS *ADDITIONAL PINS OMITTED FOR CLARITY RFS SCLK DR RCLK DR CONVST RFS CONTROL TIMER AD7868* 4.7k Ω 2k Ω 4.7k Ω 5V – 5V + Figure 17. AD7868—ADSP-2101/ADSP-2102 Interface |
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