7.3.14 FSK Modulation
Direct digital FSK modulation is supported in LMX2572LP. FSK modulation is achieved by changing the output frequency by changing the N divider value. The LMX2572LP supports three different types of FSK operation.
- FSK SPI mode. This mode supports discrete 2-, 4- and 8-level FSK modulation. There are eight dedicated registers used to pre-store the desired FSK frequency deviations. Program FSK_SPI_DEV_SEL to select one of the FSK deviations at a time.
- FSK SPI FAST mode. In this mode, instead of selecting one of the pre-stored FSK deviations, change the FSK deviation directly by writing to FSK_SPI_FAST_DEV. As a result, this mode supports arbitrary-level FSK, 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. In this mode, only the 16 bits of the DATA field are required to program. The data is transmitted on the high or low side of the frame sync (programmable in register R114, 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 75 MHz. Timing diagram is shown in Figure 26 while the timing specification is same as SPI write timing as shown in the Timing Requirements section.