Texas Instruments MSP430x1xx - Manual (Page 110)

Digitally-Controlled Oscillator (DCO)
7-10
7.3 Digitally-Controlled Oscillator (DCO)
The DCO is an integrated RC-type oscillator in the Basic Clock Module. The
DCO frequency can be tuned by software using the DCO, MOD, and RSEL
bits. The DCO is absolutely monotonic. As with any RC-type oscillator, fre-
quency varies with temperature, voltage, and from device to device. The digital
control of the oscillator allows frequency stabilization despite its RC-type char-
acteristics.
Figure 7–10. DCO Schematic
1
0
XDCOR
DC Generator DCO
Modulator Halt
3
3 5
0 1 2
Rsel
DCO MOD
XSELM.1
CPUOff SCG1
SELS
V
CC
V
CC
DCOCLK
SCG0
The dc generator, when switched off, requires some minimal start-up time (4
microsecond range) due to its low-current design. Once the current is switched
on, the resistor injects current in the microampere range into the dc generator;
the internal and external parasitic capacitances introduce the delay in the
microsecond range. No delay occurs in operating modes that do not require
to switch off the dc generator current.
An internal or external resistor is connected to the dc generator, which
determines the operating fundamental frequency of the DCOCLK.
The frequency of DCOCLK is set by the following functions:
- The current injected into the dc generator (DCGEN) by either the internal
or external resistor defines the fundamental frequency. Control bit DCOR
selects the internal or external resistor.
- Control bits Rsel2, Rsel1, and Rsel0 divide the fundamental frequency
into eight nominal frequency ranges. These ranges are defined for an
individual device in the appropriate data sheet.
- The three control bits, DCO0 to DCO2, adjust the DCOCLK frequency.
- The five modulation bits, MOD0 to MOD4, switch between the frequency
selected by the DCO bits and the next higher frequency set by DCO+1.
The clock period of the DCOCLK signal changes approximately ten percent
for each step of the control bit DCO.