On Mon, 14 Jan 2008 02:00:33 -0500, ASAAR <caught@[EMAIL PROTECTED]
> wrote:
>On Sun, 13 Jan 2008 22:02:40 GMT, Steve wrote:
>
>>> With SD using serial data transfer, it may be a point in favor of
>>>CF cards, which should then have the potential to have a much
>>>greater data transfer rate. For me though, SD is fast enough. But
>>
>> Serial data transfer has much higher potential data rates. I can get
>> into the technical reasons why if you want. But if you feel like
>> doing your own research, just look up why SATA is faster than PATA.
>
> Though it might be interesting, it's not worth the effort right
>now, for reasons mentioned here not too long ago. I assume it has
>something to do with the serial stream zipping along as fast as
>possible (with possibly a clock signal), vs. parallel data having to
>settle and remain stable long enough to be sampled. Do you know
Pretty much right. Signal paths for the data lines, switch times for
the digital circuits, etc., all become very critical as parallel rates
are pushed up. Circuit board layout is complicated to keep all the
data lines the same length for the entire path from controller to
controller. None of that is critical for serial. The only thing
critical for serial is standard high frequency design practices.
>whether SD (or SDHC) serial rates are high enough to exceed CF data
>rates? From what I've read of tests performed within the last year
>or so, Sandisk Extreme IV CF cards were the fastest, although these
>had to use compatible Sandisk readers to achieve the fastest rates.
For both current CF and SD real-world data transfer, the interface
speed is not the limiting factor.
CF+ 2.0 is 16MB/s, 3.0 is 66MB/s and CF+ 4.0 interface is 133MB/s.
SD/MMC interface is a bit more complicated since it can operate as a 1
bit serial, or in 4 or 8 bit modes. In 1 bit serial, max rate for SD
is 6.25MB/s, MMC+ is 6.5MB/s. SD in 4 bit mode is 25MB/s. MMC+ in 4
or 8 bit modes is 26MB/s or 52MB/s.
The reason SD seems so much slower in 1 bit mode than CF is because SD
uses a slow serial clock speed of 50MHz max, and a lot of hardware
only sup****ts 25MHz. SD could always bump up the clock rates if
faster flash memory made the interface the choke point. They'll have
to do that if they want to go much faster than the "133x" memory
speeds that are becoming common today.
CF is pretty close to it's practical limit at 133MB/s. But unlike
current SD, that should be fast enough for a long time.
Steve
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