Texas Instruments 3138 155 232931 manual Crystal Oscillator

The battery monitor is controlled through the BATTMON control register. The battery monitor is enabled and disabled using the BATTMON.BATTMON_EN control bit. The voltage regulator must also be enabled when using the battery monitor.

The battery monitor status bit is available in the BATTMON.BATTMON_OK status bit. This bit is high when the VREG_IN input voltage is higher than the toggle voltage

Vtoggle.

The battery monitor toggle voltage is set in the 5-bit BATTMON.BATTMON_VOLTAGE control bits. BATTMON_VOLTAGE is an unsigned, positive number from 0 to 31. The toggle voltage is given by:

Vtoggle = 1.25 V⋅	72 − BATTMON_VOLTAGE
	27

31 Crystal Oscillator

An external clock signal or the internal crystal oscillator can be used as main frequency reference. The reference frequency must be 16 MHz. Because the crystal frequency is used as reference for the data rate as well as other internal signal processing functions, other frequencies cannot be used.

If an external clock signal is used this should be connected to XOSC16_Q1, while XOSC16_Q2 should be left open. The MAIN.XOSC16M_BYPASS bit must be set when an external clock signal is used.

Using the internal crystal oscillator, the crystal must be connected between the XOSC16_Q1 and XOSC16_Q2 pins. The oscillator is designed for parallel mode operation of the crystal. In addition, loading capacitors (C381 and C391) for the crystal are required. The loading capacitor values depend on the total load capacitance, CL, specified for the crystal. The total load capacitance seen between the crystal terminals should equal CL for the crystal to oscillate at the specified frequency.

CC2420

Alternatively, for a desired toggle voltage, BATTMON_VOLTAGE should be set according to:

BATTMON_VOLTAGE = 72 − 27 ⋅ Vtoggle

1.25 V

The voltage regulator must be enabled for at least 100 µs before the first measurement. After being enabled, the BATTMON_OK status bit needs 2 µs to settle for each new toggle voltage programmed.

The main performance characteristics of the battery monitor is shown in the Electrical Specifications section on page 9.

CL = 1 1 1 + Cparasitic

C381 + C391

The parasitic capacitance is constituted by pin input capacitance and PCB stray capacitance. The total parasitic capacitance is typically 2 pF - 5 pF.

The crystal oscillator circuit is shown in Figure 30. Typical component values for different values of CL are given in Table 10.

The crystal oscillator is amplitude regulated. This means that a high current is used to start up the oscillations. When the amplitude builds up, the current is reduced to what is necessary to maintain a stable oscillation. This ensures a fast start-up and keeps the drive level to a minimum. The ESR of the crystal must be within the specification in order to ensure

areliable start-up (see the Electrical Specifications section).

SWRS041B Page 53 of 89