|ROM API functions / Boot ROM|
During the boot process, the boot loader checks if there is valid user code in flash. The criterion for valid user code is as follows:
The reserved Cortex-M0+ exception vector location 7 (offset 0x0000 001C in the vector table) should contain the 2’s complement of the check-sum of table entries 0 through 6. This causes the checksum of the first 8 table entries to be 0. The bootloader code checksums the first 8 locations in sector 0 of the flash. If the result is 0, then execution control is transferred to the user code.
If the signature is not valid, the auto-baud routine synchronizes with the host via serial port USART0. The host should send a ’?’ (0x3F) as a synchronization character and wait for a response. The host side serial port settings should be 8 data bits, 1 stop bit and no parity. The auto-baud routine measures the bit time of the received synchronization character in terms of its own frequency (the 12 MHz IRC frequency) and programs the baud rate generator of the serial port. It also sends an ASCII string ("Synchronized<CR><LF>") to the host. In response, the host should send the same string ("Synchronized<CR><LF>").
The boot loader auto-baud routine looks at the received characters to verify synchronization. If synchronization is verified then "OK<CR><LF>" string is sent to the host. The host should respond by sending the crystal frequency (in kHz) at which the part is running. The response is required for backward compatibility of the boot loader code and, on the LPC81x, is ignored. The boot loader configures the part to run at the 12 MHz IRC frequency.
Once the crystal frequency response is received, the part is initialized and the ISP command handler is invoked. For safety reasons an "Unlock" command is required before executing the commands resulting in flash erase/write operations and the "Go" command. The rest of the commands can be executed without the unlock command. The Unlock command is required to be executed once per ISP session. The Unlock command is explained in ISP-IAP API UART ISP Unlock command.
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