CYW20715 Datasheet, PDF (23/47 Pages) Cypress Semiconductor – Bluetooth 4.0 + EDR compliant

CYW20715
TCXO OR Option
The CYW20715 has an optional feature that allows the application to perform a logical OR function on a system TCXO clock request
signal and the CYW20715 clock request to form one clock request output to the TCXO device. This logical OR function is embedded
in the pad ring so that it is available at any time, as long as the pad ring is receiving a VDDO supply. The function works even if the
CYW20715’s digital core is sleeping or completely powered off.
TTable 4 shows the truth table.
Table 4. Truth Table
GPIO_6
CLK_REQ_IN
0
0
1
1
Internal Clock Request State
(0 = sleep)
0
1
0
1
GPIO_5
CLK_REQ
0
1
1
1
6.4 Frequency Selection
Any frequency within the range specified for the crystal and TCXO reference can be used. These frequencies include standard handset
reference frequencies (12, 13, 14.4, 15.36, 16.2, 16.8, 18, 19.2, 19.44, 19.68, 19.8, 20, 24, 26, 33.6, 37.4, and 38.4 MHz) and any
frequency between these reference frequencies, as desired by the system designer. Since bit timing is derived from the reference
frequency, the CYW20715 must have the reference frequency set correctly in order for the UART and PCM interfaces to function
properly.
The CYW20715 reference frequency can be set in one of three ways.
■ Use the default 20 MHz frequency
■ Designate the reference frequency in external NVRAM
■ Auto-detect the standard handset reference frequencies using an external LPO clock
The CYW20715 is set to a default frequency of 20 MHz at the factory. For a typical design using a crystal, it is recommended that the
default frequency be used, since this simplifies the design by removing the need for either external NVRAM or external LPO clock.
If the application requires a frequency other than the default, the value can be stored in an external NVRAM. Programming the
reference frequency in NVRAM provides the maximum flexibility in the selection of the reference frequency, since any frequency within
the specified range for crystal and external frequency reference can be used. During power-on reset (POR), the device downloads
the parameter settings stored in NVRAM, which can be programmed to include the reference frequency and frequency trim values.
Typically, this is how a PC Bluetooth application is configured.
For applications such as handsets and portable smart communication devices, where the reference frequency is one of the standard
frequencies commonly used, the CYW20715 automatically detects the reference frequency and programs itself to the correct reference
frequency. In order for auto-frequency detection to work properly, the CYW20715 must have a valid and stable 32.768 kHz external LPO
clock present during POR. This eliminates the need for NVRAM in applications where the external LPO clock is available and an external
NVRAM is typically not used.
6.5 Frequency Trimming
The CYW20715 uses a fractional-N synthesizer to digitally fine-tune the frequency reference input to within ±2 ppm tuning accuracy.
This trimming function can be applied to either the crystal or an external frequency source such as a TCXO. Unlike the typical crystal-
trimming methods used, the CYW20715 changes the frequency using a fully digital implementation and is much more stable and
unaffected by crystal characteristics or temperature. Input impedance and loading characteristics remain unchanged on the TCXO or
crystal during the trimming process and are unaffected by process and temperature variations.
The option to use or not use frequency trimming is based on the system designer’s cost trade-off between bill-of-materials (BOM) cost
of the crystal and the added manufacturing cost associated with frequency trimming. The frequency trimming value can either be
stored in the host and written to the CYW20715 as a vendor-specific HCI command or stored in NVRAM and subsequently recalled
during POR.
Frequency trimming is not a substitute for the poor use of tuning capacitors at an crystal oscillator (XTAL). Occasionally, trimming can
help alleviate hardware changes.
Document Number: 002-14813 Rev. *B
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