Lpc2000

Hi Leighton

As you are now aware the Philips ISP generate the check sum during 
download. However there may be some more issues here to consider.

1. The checksum is used by the boot to check whether there is user 
code in the FLASH. If the check is not good the boot code stays in 
ISP mode thus allowing an update. The value must be correct 
otherwise the (user) code will never start.

2. The validity of the check sum says nothing about the validity of 
the program. A bad program can crash as soon as it starts and the 
boot software is stuck. It will always start the bad code and this 
will always crash - to get out of this loop the only way is to hold 
the ISP pin input low on reset.

3. Also the download of a good software is not fool proof. If the 
download is interrupted before it is complete you effectively get 
situation 2.

4. An improvement of the Philips ISP loader would be to load the 
application coded first and the vector tables with check sum 
(validity) as very last job. This would avoid 2 occuring when 
interrupting a download in 99,99% of the casees (exception is 
interrupting as the vactor table exactly then when it is being 
programmed...)

Cheers

Mark Butcher
www.mjbc.ch

PS I have an encoded signature over the user program which is 
verified before the application allows itself to start (security 
requirement). Important to note is that this technique requires 
about 32k of the code space just for this feature - is it worth it 
in most applications? Code can be used from OpenSSL project...



--- In lpc2000@yahoogroups.com, "Leighton Rowe" <leightonsrowe@y...> 
wrote:
> 
> Is it true that Phillips ISP does the checksum calculation before 
> programming the vector code into flash?
> 
> I notice the .hex files that I'm making (using IAR's software) 
> doesn't contain the calculated checksum (0x14)for the vector code. 
> It's usually filled with 0's (without the fill option).
> 
> Is there's a way for me to generate the vector checksum in the 
0x14 
> space of the hex file?