IC

IC
The Atmel AT89S microcontrollers use three SPI lines to
implement in-system programming (ISP). When the ISP
programmer asserts the RESET line, the target AT89S
microcontroller is supposed to tri-state all I/O pins including
the SPI lines and then wait for commands from the programmer.
Experiments using AT89S devices have shown
that if the user code writes to any of the ISP I/O pins
(MOSI, MISO, SCK) within approximately 500 ms of the
user code coming out of RESET condition, any subsequent
serial programming operation fails. The problem is then
that the only way to re-program the device successfully is
to physically remove it from the target board and erase it in
a parallel programmer.

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