Errata Sheet

2

ISP Line Driving Contention

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 pro-

grammer. 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.

The Solution

Make sure that the user code does not access any of the

ISP I/O pins within 500 ms of coming out of RESET

condition.