[linux-dvb] qam256 problem with Technotrend C2300 DVB-C card

e9hack e9hack at googlemail.com
Sat Nov 18 11:07:58 CET 2006

Andrew de Quincey wrote:
> On Sunday 08 October 2006 08:43, e9hack wrote:
>> Hi,
>> I've problems with the TT-C2300 DVB-C card and qam256 modulated channels.
>> After switching to a qam256 channel, the card needs between 0.5 and 1 hour
>> to get the lock. Possible it is a problem of the reduced i2c bus speed. If
>> the stv0297 is accessed with a higher i2c bus speed, the card gets the lock
>> within a few seconds like qam64. I've attach a patch, which does reduce the
>> i2c bus speed for the tuner only. I've measured the time between 'after
>> tuning' and 'start waiting for the agc lock' in stv0297_set_frontend().
>> Without the patch it needs 470ms and with the patch 60ms. This is a little
>> bit strange, because the factor of the speed changing is two. Possible,
>> there is a bug in the spec of the saa7146.
> Looks sensible to me; no point slowing the demod access just for the tuner. 
> Will commit once you send me a signed off by line for it...
The i2c-bus speed problem is already fixed by
http://linuxtv.org/hg/v4l-dvb?cs=4a4213dbb9b8;style=gitweb. It was a
problem of the max loop count in saa7146_i2c_writeout(). The saa7146 can
transfer three bytes over the i2c-bus with one upload. The TT-C2300 uses
a reduced i2c speed of 137.5kHz. This is necessary, because the pll chip
(tda6405) has a max i2c-speed of 150kHz. The time for a transfer of
three bytes is ca. 200us. With the old max loop count of 20, I got an
additional delay of 1ms for the half of the transfers. This has
increased the initialization time of the stv0297.

I think. it isn't necessary to commit this patch.

- Hartmut

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