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ATA5749-6DQ Datasheet(PDF) 4 Page - ATMEL Corporation |
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ATA5749-6DQ Datasheet(HTML) 4 Page - ATMEL Corporation |
4 / 26 page 4 9128D–RKE–01/09 ATA5749 [Preliminary] 3. Functional Description 3.1 Fractional-N PLL The ATA5749 block diagram is shown in Figure 1-1 on page 2. The operation of the PLL is determined by the contents of a 32-bit configuration register. The 15-bit value FREQ is used with the 1-bit 434_N315 flag to determine the RF carrier frequency. This results in a user-selectable frequency step size of 793 Hz (with 13.000 MHz crystal). With this level of resolution, it is possi- ble to compensate for crystal tolerance by adjusting the value of FREQ accordingly. This enables the use of lower cost crystals without compromising final accuracy. In addition, software programming of RF carrier frequency allows this device to be used in some multi-channel applications. Modulation type is selected with the 1-bit ASK_NFSK flag. FSK modulation is achieved by mod- ifying the divider block in the feedback loop. The benefit to this approach is that performance- reducing RF spurs (common in applications that create FSK by “pulling” the load capacitance in the crystal oscillator circuit) are completely eliminated. The 8-bit value FSEP establishes the FSK frequency deviation. It is possible to obtain FSK frequency deviations from ±396 Hz to ±101 KHz in steps of ±396 Hz. The PLL lock time is 1280/(external crystal frequency) and amounts to 98.46 µs when using a 13.0000 MHz crystal. When added to the crystal oscillator start-up time, a very fast time-to-transmit is possible (typically 300 µs). This feature extends battery life in applications like Tire Pressure Monitoring Systems, where the message length is often shorter than 10 ms and the time “wasted” during start-up and settling time becomes more significant. 3.2 Selecting the RF Carrier Frequency The fractional divider can be programmed to generate an RF output frequency f RF according to the formulas shown in Table 3-1. Note that in the case of f RF ASK, the FSEP/2 value is rounded down to the next integer value if FSEP is an odd number. FSEP can take on the values of 1 to 255. Using a 13.000 MHz crystal, the range of frequency deviation f DEV_FSK is programmable from ±396 Hz to ±101.16 kHz in steps of ±396 Hz. For example, with FSEP = 100 the output frequency is FSK modulated with f DEV_FSK = ±39.6 kHz. FREQ can take values in the range of values 2500 and 22000. Using a 13.0000 MHz crystal, the output frequency f RF can be program med to 315 MHz by setting F REQ[0:14] = 3 730, FSEP[0:7] = 100 and S434_N315 = 0. By setting FREQ[0:14] = 14342, FSEP[0:7] = 100 and S434_N315 = 1, 433.92 MHz can be realized. Table 3-1. RF Output Parameter Formulas RF Output Parameter S434_N315 = LOW S434_N315 = HIGH fRF_FSK_LOW (24 + (FREQ + 0.5)/16384) × f XTO (32.5 + (FREQ + 0.5)/16384) × f XTO fRF_FSK_HIGH (24 + (FREQ + FSEP + 0.5)/16384) × f XTO (32.5 + (FREQ + FSEP + 0.5)/16384) × f XTO f DEV__FSK FSEP/32768 × f XTO FSEP/32768 × f XTO fRF ASK (24 + (FREQ + FSEP/2 + 0.5)/16384) × f XTO (32.5 + (FREQ + FSEP/2 + 0.5)/16384) × f XTO |
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