The flash boot loader code is executed every time the part is powered on or reset. The
loader can execute the ISP command handler or pass execution to the user application
code.
A LOW level, after reset, at the P0.14 pin is considered as the external hardware request
to start the ISP command handler. The boot loader samples this pin during reset.
Assuming that proper signal is present on X1 pin when the rising edge on RST pin is
generated, it may take up to 3 ms before P0.14 is sampled and the decision on whether to
continue with user code or ISP handler is made. If P0.14 is sampled LOW and the
watchdog overflow flag is set, the external hardware request to start the ISP command
handler is ignored. If there is no request for the ISP command handler execution (P0.14 is
sampled HIGH after reset), a search is made for a valid user program. If a valid user
program is found then the execution control is transferred to it. If a valid user program is
not found, the auto-baud routine is invoked.
Pin P0.14 is used as hardware request for ISP requires special attention. Since P0.14 is in
high impedance mode after reset, it is important that the user provides external hardware
(a pull-up resistor or other device) to put the pin in a defined state. Otherwise unintended
entry into ISP mode may occur.