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MX7705 Datasheet(PDF) 21 Page - Maxim Integrated Products |
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MX7705 Datasheet(HTML) 21 Page - Maxim Integrated Products |
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21 / 34 page  External Oscillator The oscillator requires time to stabilize when enabled. Startup time for the oscillator depends on supply voltage, temperature, load capacitances, and center frequency. Depending on the load capacitance, a 1M Ω feedback resistor across the crystal can reduce the startup time (Figure 7). The MX7705 was tested with an ECS-24-32-1 (2.4576MHz crystal) and an ECS-49-20-1 (4.9152MHz crystal) (see the Typical Operating Characteristics). In power-down mode, the supply current with the external oscillator enabled is typically 67µA with a 3V supply and 227µA with a 5V supply. Serial-Digital Interface The MX7705 interface is fully compatible with SPI-, QSPI-, and MICROWIRE-standard serial interfaces. The serial interface provides access to seven on-chip registers. The registers are 8, 16, and 24 bits in size. Drive CS low to transfer data in and out of the MX7705. Clock in data at DIN on the rising edge of SCLK. Data at DOUT changes on the falling edge of SCLK and is valid on the rising edge of SCLK. DIN and DOUT are trans- ferred MSB first. Drive CS high to force DOUT high impedance and cause the MX7705 to ignore any signals on SCLK and DIN. Connect CS low for 3-wire operation. Figures 8 and 9 show the timings for write and read operations, respectively. On-Chip Registers The MX7705 contains seven internal registers (Figure 10), which are accessed by the serial interface. These regis- ters control the various functions of the device and allow the results to be read. Table 7 lists the address, power-on default value, and size of each register. The first of these registers is the communications register. The 8-bit communications register controls the acquisition channel selection, whether the next data transfer is a read or write operation, and which register is to be accessed. The second register is the 8-bit setup register, which con- trols calibration modes, gain setting, unipolar/bipolar inputs, and buffered/unbuffered modes. The third register is the 8-bit clock register, which sets the digital filter char- acteristics and the clock control bits. The fourth register is the 16-bit data register, which holds the output result. The 24-bit offset and gain registers store the calibration coeffi- cients for the MX7705. The 8-bit test register is used for factory testing only. The default state of the MX7705 is to wait for a write to the communications register. Any write or read opera- tion on the MX7705 is a two-step process. First, a com- mand byte is written to the communications register. This command selects the input channel, the desired register for the next read or write operation, and whether the next operation is a read or a write. The sec- ond step is to read from or write to the selected regis- ter. At the end of the data-transfer cycle, the device returns to the default state. See the Performing a Conversion section for examples. If the serial communication is lost, write 32 ones to the serial interface to return the MX7705 to the default state. The registers are not reset after this operation. 16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC ______________________________________________________________________________________ 21 CRYSTAL OR CERAMIC RESONATOR CL CL CLKIN CLKOUT OPTIONAL 1M Ω MX7705 Figure 7. Using a Crystal or Ceramic Oscillator CS t2 t6 t9 t10 SCLK MSB LSB DIN Figure 8. Write Timing Diagram t2 t4 t5 t3 t8 t6 t7 t1 SCLK MSB LSB DOUT CS DRDY Figure 9. Read Timing Diagram |
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