Maxim Integrated MAX32665 - 8 Khz Ultra-Low Power Nano-Ring Internal Oscillator; 8 Mhz Internal Oscillator; Figure 4-2: Example 32 Mhz Crystal Capacitor Determination; Khz External Crystal Oscillator

MAX32665-MAX32668 User Guide
Maxim Integrated Page 53 of 457
Figure 4-2: Example 32MHz Crystal Capacitor Determination
32MHz
CRYSTAL
HFXIN
HFXOUT
MAX32665MAX32668
C
LOUT
C
LIN
C
STIN
C
STOUT
C
HFX_PIN
C
HFX_PIN
C
STRAYIN
= C
STIN
+ C
HFX_PIN
C
STRAYOUT
= C
STOUT
+ C
HFX_PIN
C
HFXOUT
= C
STRAYOUT
+ C
LOUT
C
HFXIN
= C
STRAYIN
+ C
LIN
The crystal load, C
L
, as specified in the MAX32665-MAX32668 datasheet Electrical
Characteristics Table is required to be 12pF. Therefore, the total capacitance se en
by the crystal must equal C
L
.
C
L
= (C
HFXIN
X C
HFXOUT
) / (C
HFXIN
+ C
HFXOUT
)
Assume that C
LIN
= C
LOUT
.
Assume the device pin capacitance of the HFXOUT and HFXIN pins respectively is =
4pF each. This specification is outlined in the Electrical Characteristics Tab le of the
MAX32665-MAX32668 datasheet as C
HFX_PIN
.
Assume the circu it board stray capacitance represented in the diagram by C
STIN
and
C
STOUT
= 0.5pf each.
Solve for C
LOUT
,
C
LOUT
= 19.5pF = C
LIN
Choose 20pF for C
LOUT
= C
LIN
4.1.5 7.3728MHz Internal Oscillator
This is a very low power internal oscillator that can be selected as SYS_OSC.
This clock can optionally be used as a dedicated baud rate clock for the UARTs. This is useful if the SYS_OSC selected does
not allow the targeted UART baud rate.
Firmware selection of the voltage that controls this oscillator is controlled by the register bit GCR_CLK_CTRL.hirc7m_vs. The
internal CPU core supply voltage (V
CORE
) is the default option. The external pin V
DDA
can also be selected. The V
DDA
pin goes
to an internal 1V regulator that also provides the analog supply voltage for this device.
This oscillator can optionally be automatically powered down when in DEEPSLEEP mode by setting register bit
GCR_PMR.hirc7mpd.
This oscillator is disabled by default at power-up.
4.1.6 32.768kHz External Crystal Oscillator
This is a very low power internal oscillator that can be selected as SYS_OSC. This oscillator can optionally use a 32.768kHz
input clock instead of an external crystal. The internal 32.768kHz clock is available as an output on GPIO as an alternate
function (SQWOUT).
This oscillator is the dedicated clock for the Real-Time Clock (RTC). If the RTC is enabled, the 32.768kHz external oscillator
must be enabled, independent of the selection of SYS_OSC. This oscillator is disabled at power-up.
4.1.7 8kHz Ultra-Low Power Nano-Ring Internal Oscillator
This is an ultra-low power internal oscillator that can be selected as SYS_OSC.
This oscillator is enabled at power-up and cannot be disabled by firmware.