Hi,
I want to clarify how xtal_trim value is calculated now.
First of all the xtal_mode set to 0x5 will require a 16 Mhz clock signal at
pin 26. I don't have such at86rf2xx board which can do that, so I don't want
to support it right now.
IMPORTANT:
The xtal_mode for external frequency differs per at86rf2xx transceiver. For
at86rf230/at86rf231 it's 0x4. For at86rf233 it's 0x5. (Thanks Werner
Almesberger who pointed this out). That doesn't matter right now, because we
don't change the "normal" mode.
The xtal_trim is only necesarry for external crystal, when xtal_mode == 0xF.
The xtal_trim value is calculated by:
CL = capacitor of used crystal
CX = connected capacitors at xtal pins
CPAR = in all at86rf2xx datasheets this is a constant value 3 pF,
but this is different on each board setup. You need to fine
tuning this value via CTRIM.
CTRIM = variable capacitor setting. Resolution is 0.3 pF range is
0 pF upto 4.5 pF.
CL = 0.5 * (CX + CTRIM + CPAR)
Examples:
On atben [0]:
CL = 8 pF
CX = 12 pF
CPAR = 3 pF (We assume the magic constant from datasheet)
CTRIM = 0.9 pF
(12+0.9+3)/2 = 7.95 which is nearly at 8 pF
openlabs [1]:
CL = 16 pF
CX = 22 pF
CPAR = 3 pF (We assume the magic constant from datasheet)
CTRIM = 4.5 pF
(22+4.5+3)/2 = 14.75 which is the nearest value to 16 pF
For more information it's the section "Integrated Oscillator Setup" inside
the at86rf2xx datasheets. (In my case 8111C–MCU Wireless–09/09).
For a better calculation of the CPAR value, Werner Almesberger developed some
diagnostic tool [2], which I don't tried out yet.
- Alex
[0] http://projects.qi-hardware.com/index.php/p/ben-wpan/source/tree/master/atben
(atben.sch, requires kicad tool)
[1] http://openlabs.co/OSHW/Raspberry-Pi-802.15.4-radio-files/rpi802154-r1.pdf
[2] http://projects.qi-hardware.com/index.php/p/ben-wpan/source/tree/master/tools/atrf-xtal
changes since v4:
- remove platform data from at86rf230_local
- add note in cover-letter because at86rf2xx have different xtal_mode values
changes since v3:
- remove setting of xtal_mode. Instead we setting xtal_trim only.
changes since v2:
- copy platform data to driver allocated space
Cc: Werner Almesberger <werner@xxxxxxxxxxxxxxx>
Cc: Thomas Stilwell <stilwellt@xxxxxxxxxxx>
Alexander Aring (2):
at86rf230: copy pdata to driver allocated space
at86rf230: add support for external xtal trim
.../bindings/net/ieee802154/at86rf230.txt | 3 +
drivers/net/ieee802154/at86rf230.c | 66 +++++++++++++---------
include/linux/spi/at86rf230.h | 1 +
3 files changed, 42 insertions(+), 28 deletions(-)
--
2.3.0
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