Direct digital FSK modulation is supported in
LMX2571-EP. FSK modulation is achieved by changing the output frequency by changing
the N-divider value. The LMX2571-EP supports four different types of FSK
operation.
- FSK PIN mode. LMX2571-EP supports 2-, 4-, and 8-level FSK modulation in PIN mode. In this mode, symbols are directly fed to the FSK_D0, FSK_D1, and FSK_D2 pins. Symbol clock is fed to the FSK_DV pin. Symbols are latched into the device on the rising edge of the symbol clock. The maximum supported symbol clock rate is 1 MHz. The device has eight dedicated registers to prestore the desired FSK frequency deviations, with each register corresponding to one of the FSK symbols. The LMX2571-EP will change its output frequency according to the states on the FSK pins; no extra register programming is required.
- FSK SPI mode. This mode is identical to the FSK PIN mode with the exception that the control for the selected FSK level is not performed with external pins but with register R34. Each time when register R34 is programmed, change only the FSK_DEV_SEL field to select the desired FSK frequency deviation as stored in the dedicated registers.
- FSK SPI FAST mode. In this mode, instead of selecting one of the prestored FSK level, change the FSK deviation directly by writing to the register R33, FSK_DEV_SPI_FAST field. As a result, this mode supports arbitrary-FSK level, which is useful to construct pulse-shaping or analog-FM modulation.
- FSK I2S mode. This mode is similar to the FSK SPI FAST mode, but the programming format is an I2S format on dedicated pins instead of SPI. The benefit of using I2S is that this interface could be shared and synchronous to other digital audio interfaces. The same FSK data input pins that are used in FSK PIN mode are reused to support I2S programming. In this mode only the 16 bits of DATA field is required to program. The data is transmitted on the high or low side of the frame sync (programmable in register R34, FSK_I2S_FS_POL). The unused side of the frame sync needs to be at least one clock cycle. In other words, 17 (16 + 1) CLK cycles are required at a minimum for one I2S frame. Maximum I2S clock rate is 100 MHz.
Figure 7-4 FSK PIN Mode Timing Figure 7-5 FSK I2S Mode Timing See Section 8.1.1 for FSK operation details.