Texas Instruments MSP430x1xx manual DCO Modulator, 7. Modulator Patterns

Basic Clock Module Operation

4.2.5DCO Modulator

The modulator mixes two DCO frequencies, fDCO and fDCO+1 to produce an intermediate effective frequency between fDCO and fDCO+1 and spread the

clock energy, reducing electromagnetic interference (EMI). The modulator

mixes fDCO and fDCO+1 for 32 DCOCLK clock cycles and is configured with the MODx bits. When MODx = 0 the modulator is off.

The modulator mixing formula is:

t =(32− MODx) ⋅ tDCO + MODx ⋅ tDCO+1

Because fDCO is lower than the effective frequency and fDCO+1 is higher than the effective frequency, the error of the effective frequency integrates to zero.

It does not accumulate. The error of the effective frequency is zero every 32 DCOCLK cycles. Figure 4−7 illustrates the modulator operation.

The modulator settings and DCO control are configured with software. The DCOCLK can be compared to a stable frequency of known value and adjusted with the DCOx, RSELx, and MODx bits. See http://www.msp430.com for application notes and example code on configuring the DCO.

Figure 4−7. Modulator Patterns

MODx
31
24
16
15
5
4
3
2	Upper DCO Tap Frequency fDCO+1
Lower DCO Tap Frequency fDCO
1
0

Basic Clock Module 4-9