Figure 1 presents a Design Idea for a USB-based data-acquisition system that uses a serial ADC employing general-purpose components, such as D flip-flops, a binary counter, and a shift register. Using the DLP-USB245M FIFO-to-USB-converter module from DLP Design (www.dlpdesign.com), you can communicate with the peripheral device through the USB port of a host computer. You can write your own program to read and write the data through this module or simply download free test-application software available from DLP’s Web site. Additionally, you could download National Instruments’ (www.ni.com) LabView serial-read and-write VIs (virtual instruments).
Writing a dummy block of data from the host computer to the buffer of the DLP-USB245M generates a spike at the module’s pin, which triggers the D flip-flop, FF2 of the 74LS74. The flip-flop’s pin initiates the conversion cycle of the MAX187 serial ADC from Maxim (www.maxim-ic.com) by pulling down its chip-select pin. The ADC’s end-of-conversion cycle causes a low-to-high transition from its DOUT pin, which triggers the other D flip-flop, FF1 of the 74LS74, to generate a gating pulse, Q1, for the serial-clock pulses that read the data from the same DOUT pin of the ADC. The 74LS90 binary counter counts the serial-clock pulses. When the count reaches nine, the counter resets the gating pulse for the serial clock and pushes back the chip-select signal to a high level by resetting both FF1 and FF2, ending the ADC’s acquisition cycle.
The system acquires the data at the falling edge of the MAX187’s SCLK pin and shifts it into the 74LS164 serial-to-parallel shift register at the rising edge of the next SCLK. The MAX187 needs nine serial-clock pulses to shift valid 8-bit data. This circuit uses only 8 bits of the 12-bit ADC. If the circuit requires all 12 bits, then you must connect all NAND gates at the appropriate outputs of the binary counter to generate a reset signal by its 13th clock pulse, and you must make the shift register larger.
The serial data from the ADC converts to parallel data in the serial-to-parallel shift register; a WR (write) signal to the DLP-USB245M then transfers this data to the PC. This action is a complement of the signal from Q2 of the 74LS74. The DLP-USB245M’s pin generates a trigger to initiate the conversion cycle and clears the previous data of the shift register.
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Figure 1 This circuit performs a serial-to-parallel conversion of serial-ADC data and transfers the data to the USB port of a PC.
Click here for the illustrations: Figure 1
