Datasheet

MODx
Lower DCO Tap Frequency f
DCO
31
24
16
15
5
4
3
2
1
0
Upper DCO Tap Frequency f
DCO+1
www.ti.com Basic Clock Module+ Operation
MOV.B &CALBC1_1MHZ,&BCSCTL1 ; Set range
MOV.B &CALDCO_1MHZ,&DCOCTL ; Set DCO step + modulation
5.2.5.3 Using an External Resistor (R
OSC
) for the DCO
Some MSP430F2xx devices provide the option to source the DCO current through an external resistor,
R
OSC
, tied to DV
CC,
when DCOR = 1. In this case, the DCO has the same characteristics as MSP430x1xx
devices, and the RSELx setting is limited to 0 to 7 with the RSEL3 ignored. This option provides an
additional method to tune the DCO frequency by varying the resistor value. See the device-specific data
sheet for parameters.
5.2.6 DCO Modulator
The modulator mixes two DCO frequencies, f
DCO
and f
DCO+1
to produce an intermediate effective frequency
between f
DCO
and f
DCO+1
and spread the clock energy, reducing electromagnetic interference (EMI). The
modulator mixes f
DCO
and f
DCO+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) × t
DCO
+ MODx × t
DCO+1
Because f
DCO
is lower than the effective frequency and f
DCO+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 5-7 shows 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 5-7. Modulator Patterns
5.2.7 Basic Clock Module+ Fail-Safe Operation
The basic clock module+ incorporates an oscillator-fault fail-safe feature. This feature detects an oscillator
fault for LFXT1 and XT2 as shown in Figure 5-8. The available fault conditions are:
• Low-frequency oscillator fault (LFXT1OF) for LFXT1 in LF mode
279
SLAU144J–December 2004–Revised July 2013 Basic Clock Module+
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