DW1000 User Manual
© Decawave Ltd 2017
Version 2.12
Page 202 of 242
by switching in internal capacitor banks in parallel with the external loading capacitors associated with the
chosen crystal. This trimming can be used to reduce the crystal initial frequency error and to compensate for
temperature and aging drift, if required.
The amount of trimming is programmable through
Sub-Register 0x2B:0E – FS_XTALT
8.1.1 Calibration Method
The 38.4 MHz crystal’s frequency of oscillation cannot be directly observed because it is not possible to
probe the XTAL1 and XTAL2 input pins without disturbing the oscillation. The method used is to infer the
crystal oscillation frequency by observing the output of the transmitter using a spectrum analyzer or
frequency counter.
The DW1000 is configured to transmit a continuous wave (CW) signal at a particular channel frequency. It is
suggested that channel 5 is used because the higher the frequency the easier it is to observe step changes in
the output. While accurately measuring the centre frequency of the transmission, the trim value is adjusted
until the output frequency is as close as possible to the desired frequency (e.g. 6489.6 MHz for channel 5).
In order to allow the internal trimming circuits as large a range of programming as possible, the external
loading capacitors should be chosen during the board design phase such that on average (tested on a
number of boards) the crystal trim register setting is set close to mid-range which is a 5-bit decimal value of
15or 0x0F.
Test Setup:
Reset the device so that it is in a known state before initiating the test procedure.
Configure the transmitter as described in the following steps and then monitor the RF output on a spectrum
analyser or frequency counter.
Transmitter Configuration Procedure:
2.
Write 0x00000000 to
Sub-Register 0x36:04 – PMSC_CTRL1
3.
Write the correct value for the selected channel to
Sub-Register 0x28:0C– RF_TXCTRL
, see table 37,
e.g. 0x1E3FE0 for channel 5.
4.
Register file: 0x1E – Transmit Power Control
, such as that for the
Table 20: Reference values Register file: 0x1E – Transmit Power Control.
5.
Write the value for the selected channel to
Sub-Register 0x2B:07 – FS_PLLCFG
, e.g. 0x0800041D for
Table 43: Sub-Register 0x2B:07 – FS_PLLCFG values
6.
Write the correct value for the selected channel to
Sub-Register 0x2B:0B – FS_PLLTUNE
, e.g. 0xBE for
channel 5. See table 43
7.
Write 0x22 to PMSC_CTRL0 to enable the appropriate clocks, see
8.
Write 0x00000000 to reserved register
Sub-Register 0x36:26 – PMSC_TXFSEQ
9.