Product Info

Bluegiga Technologies Oy
Page 16 of 54
4 Clock Generation
WT21 uses an internal 26 MHz crystal as a Bluetooth reference clock. All WT21 internal digital
clocks are generated using a phase locked loop, which is locked to the 26 MHz reference clock.
Also supplied to the digits is a watchdog clock, for use in low power modes. This uses a
frequency of 32.768kHz from CLK_32K, or an internally generated reference clock frequency of
1kHz, determined by PSKEY_DEEP_SLEEP_EXTERNAL_CLOCK_SOURCE.
The use of the watchdog clock is determined with respect to Bluetooth operation in low power
modes.
Bluetooth Radio
Digits
PLL
26 MHz
1 kHz
CLK_32kHz
AIO[0]
Watchdog clock
Figure 4: Clock Architecture
4.1 32kHz External Reference Clock
A 32kHz clock can be applied to CLK_32K, using
PSKEY_DEEP_SLEEP_EXTERNAL_CLOCK_SOURCE.
The CLK_32K pad is in the VDD_PADS domain with all the other digital I/O pads and is driven
between levels specified in Section 11.3.4.