RSELx _ CTR,nom
DCO,nom
DCOCONST
DCOTUNE
DCOCONST
CALCSDCOxRCAL
F
F
K
N
1
1 K
768 F
u
u
Clock System Operation
386
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Clock System (CS)
–
DCO is a source for HSMCLK (SELSx = 3).
–
DCO is a source for SMCLK (SELSx = 3).
•
For LPM3 or LPM4 or LPM3.5 or LPM4.5 mode
–
DCO is not available and is disabled. DCO_EN has no effect.
6.2.8.1
DCO Modes
The DCO can be operated in either internal resistor mode or external resistor mode. The internal resistor
mode requires no external components and is the default setting. The external resistor mode requires a
resistor to be placed at the DCOR pin to ground. This mode results in higher overall tolerance compared
to the internal resistor operation. See the device-specific data sheet for electrical characteristics. The
external resistor mode is selected by setting DCORES = 1. Switching to the external resistor mode of
operation must always be done at the default DCORSEL setting (DCORSEL = 1). Failure to do so can
result in nondeterministic behavior of the device. CSDCOERCAL0, CSDCOERCAL1 registers can be used
for calibrating the DCO in the external resistor mode of operation.
6.2.8.2
DCO Fault in External Resistor Mode
When the external resistor is detached from DCOR pin, thereby forming a open circuit condition, either
while configuring DCO in external resistor mode (DCORES: 0 to 1) or during run-time when DCO is
operating in external resistor mode, a fail-safe mechanism switches the DCO from external resistor mode
to internal resistor mode without changing the DCO frequency settings. The DCOR_OPNIFG flag is set in
such case and can be used to raise an interrupt. The CLR_DCOR_OPNIFG bit can be used to clear the
DCOR_OPNIFG flag when the DCO is configured back to internal resistor mode (DCORES = 0).
When the DCOR pin contacts ground, thereby forming a short circuit condition, either while configuring
DCO in external resistor mode (DCORES: 0 to 1) or during run-time when DCO is operating in external
resistor mode, a POR reset is triggered into the device. The DCOR_SHTIFG flag is set to indicate the
short circuit fault in the DCO external resistor configuration. The DCOR_SHT bit in the Reset Controller is
also set so that the application can determine the cause of the POR reset. The corresponding CLR bit in
the Reset Controller can be used to clear the DCOR_SHT bit in the Reset Controller as well as the
DCOR_SHTIFG bit in the Clock System.
6.2.8.3
DCO Ranges and Tuning
The DCO has six frequency ranges selected by the DCORSEL bits. Each range overlaps with its
neighboring ranges to make sure that all frequencies can be selected across the full frequency range.
Each frequency range is factory calibrated at the frequency center of the respective range. For example, if
a range is selected from 1 MHz to 2 MHz, the default center frequency is 1.5 MHz, nominal. See
for a list of available range settings. The application can select a different frequency within a range by
using the DCOTUNE bits. The DCOTUNE bits are represented in twos-complement format and represent
the offset from the center frequency. Each LSB represents an offset in the DCO period either positive or
negative depending on the sign. By default, the DCOTUNE bits are reset to 0.
Each LSB of DCOTUNE causes a change in the DCO period either positive or negative around this
nominal center period, hence decreasing or increasing the overall DCO frequency, respectively. The
nominal frequency can be computed using the specified nominal center frequency of interest, along with
its factory supplied calibration settings (available in TLV) as follows:
where
•
F
DCO,nom
= Target nominal frequency
•
F
RSELx_CTR,nom
= Calibrated nominal center frequency for DCO frequency range
x
•
K
DCOCONST
= DCO constant (floating-point value)
•
N
DCOTUNE
= DCO tune value in decimal
•
FCAL
CSDCOxRCAL
= DCO frequency calibration value for range
x
for internal or external resistor mode
(5)