Lpc2000

--- In lpc2000@yahoogroups.com, Bertrik Sikken <bertrik@z...> wrote:

> I did some experiments to get a feel for the write speed.
> 
> I measured the time it took to read and write 4MB of contiguous
> raw data directly onto the disk (without a file system). Writing
> was done both with the WRITE_BLOCK command and the
> WRITE_MULTIPLE_BLOCK.  The WRITE_MULTIPLE_BLOCK command was
> configured to write 256 sectors of 512 bytes (128 kB). I measured
> the time by looking at my wristwatch.  My hardware is a LPC2148 
> quickstart board from embedded artists, configured to run at 60
> MHz PCLK and 7.5 MHz SPI clock (highest possible for SPI0). 

Thanks for the test, this is good information.  However 
unfortunately I don't think that this is an accurate representation 
of the speed of the card itself.  Keep in mind that you have several 
limiting factors:
- 7.5MHz SPI clock (max is 25MHz, so you're 1/3rd the throughput 
there), I haven't worked with this chip yet, but why is the max 
speed 7.5MHz?
- Using SPI mode instead of SD mode.
- Optimization and organization of code

SPI mode *maximum* (for either SD or MMC) has 125KB/sec/MHz, so at 
7.5MHz your theoretical max is only 930KB/sec.

I'm not sure of how accurate the speeds reported for the cards are, 
but this may be like how my HD has a "max speed of 60MB/sec! {small 
text: burst mode from cache}", and even though it's on a bus that 
has 100MB/sec max, I only see 30MB/sec on average.

I'm also not sure if this is true, but it may also be that since 
your card is 1.5MB/sec (I assume in SPI mode and not SD 4-bit mode) 
then your card speed may be only 60KB/sec/Mhz (if it is SD) or 
75KB/sec/MHz (if it is MMC).  This translates to 450KB/sec or 
560KB/sec for SD/MMC respectively.

You're seeing about 320KB/sec. I'd guess that the code that you are 
using is not optimized (?), so it's not surprising that you're 
getting only 30% or 60% of the max speed.

Would you be able to run another test, however perhaps with the 
following modifications?

a) figure out if I'm right about the speed limit, try to benchmark 
the card speed on your computer with a USB2.0 card reader (if you 
have one).

b) use a timer algorithm in the LPC2148 to time 
- 1) the amount of time spent waiting per call to the chip
- 2) the total amount of time the read/write operation took.  

c) see if there are any simple optimizations that can be done to the 
code, or post it here and see if anyone has any ideas on how to make 
it more efficient.

It would be interesting to see what those tests show.

> My results are:
> 
> read 13s; write single 14s; write multiple 13s
> 
> I find it surprising that the write speed is not significantly 
> slower than the read speed.

Keep in mind the speed that you're operating at vs the maximum 
speed.  Let's say that on your card (at 7.5MHz) the write operation 
can occur at 400KB/sec, and the read operation at 800KB/sec.  
Because your throughput is only 320KB/sec they will both appear to 
take the same amount of time.

-- Sean