K210 can only execute the code from SRAM or from embeded ROM.

After reset K210 executes the code from embeded ROM at address 0x88000000.

Based on the state of the IO16 pin, one of two course of action can be taken:

• If the IO16 is high (not connected), ROM code copies the application code from external SPI Flash at address 0x00000000 to SRAM at address 0x80000000 and starts its execution.
• If the IO16 is low (pulled to GND), ROM code executes 1st stage ISP which waits for SLIP frames on default UART (UART3, external pins 4&5).
  An external host must issue the commands to load some valid K210 code to SRAM and start the code execution.
  Usually, the 2nd stage ISP is loaded into SRAM and executed.
  The basic purpose of the 2nd stage ISP is to provide the method of loading the application code to external SPI Flash, which will be loaded and executed in case IO16 is high at reset.
  This is how the kflash.py and similar utilities loads the firmware to SPI Flash.

The standard 2nd stage ISP used by kflash.py and similar utilities is the binary blob provided without sources and with minimal functionality: it only can load the firmware to SPI Flash.

The 2nd stage ISP provided in this repository includes all the sources and provides some enhanced functionality:

• Highly optimized, binary size less than 10KB, loads into SRAM in less than 1 second
• Write data to Flash using 4K or 64K blocks, the Flash address can be aligned to 4K or 64K.
  (standard ISP only provides 64K block writes and can only write to addresses aligned to 64K)
• Provides read from Flash function.
• In addition to chip erase function, provides also block erase function, a single sector (4KB) or 64KB block can be erased
• Provides the command to get the SPI Flash chip manufacturer's JEDEC ID, 64-bit unique serial number and size
• Provides write to SRAM and execute SRAM functions to load the application code directly to K210 SRAM and execute it.
  Similar functionality is provided by the 1st stage ISP (from ROM), but it can only load at baudrate of 115200 which makes loading large programs impractical.
  Functions from 2nd stage ISP supports loading at any baudrate supported by the used board.

Clone the repository or download and unpack the repository zip file.

The repository contains all the tools and sources needed to build 2nd stage ISP.

The same build prerequisities must be satisfied as for building any other K210 application.

• Change the working directory to the build directory.
• A simple build script BUILD.sh is provided which builds the isp binary.
• Simply execute BUILD.sh
• All files needed to use the 2nd stage ISP with kflash.py, ktool.py and similar utilities are created:
  • isp.bin the ISP binary
  • ISP_PROG.py python variable containing isp.bin ISP binary, compressed and hexlified, ready to be inserted into kflash.py, ktool.py or similar Python based application.

boris@UbuntuMate:/home/LoBo2_Razno/K210_razvoj/ktool/build$ ./BUILD.sh

 =======================================
 === Building 2ns stage ISP for K210 ===
 =======================================

=== Running 'cmake'
=== Running 'make'
Scanning dependencies of target kendryte
[  8%] Building C object lib/CMakeFiles/kendryte.dir/drivers/sysctl.c.obj
[ 33%] Building C object lib/CMakeFiles/kendryte.dir/drivers/fpioa.c.obj
[ 33%] Building C object lib/CMakeFiles/kendryte.dir/bsp/sleep.c.obj
[ 33%] Building C object lib/CMakeFiles/kendryte.dir/drivers/gpio.c.obj
[ 41%] Building C object lib/CMakeFiles/kendryte.dir/drivers/uart.c.obj
[ 50%] Building C object lib/CMakeFiles/kendryte.dir/drivers/utils.c.obj
[ 58%] Building C object lib/CMakeFiles/kendryte.dir/bsp/crt.S.obj
[ 66%] Linking C static library libkendryte.a
[ 66%] Built target kendryte
Scanning dependencies of target isp
[ 91%] Building C object CMakeFiles/isp.dir/src/isp/flash.c.obj
[ 91%] Building C object CMakeFiles/isp.dir/src/isp/main.c.obj
[ 91%] Building C object CMakeFiles/isp.dir/src/isp/slip.c.obj
[100%] Linking C executable isp
Generating .bin file ...
[100%] Built target isp
   text	   data	    bss	    dec	    hex	filename
   9907	     76	  65856	  75839	  1283f	isp
boris@UbuntuMate:/home/LoBo2_Razno/K210_razvoj/ktool/build$