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AT89C51 In-Circuit Programming
This application note illustrates the in-circuit programmability of the Atmel
AT89C51 Flash-based microcontroller. Guidelines for the addition of in-circuit
programmability to AT89C51 applications are presented along with an application
example and the modifications to it required to support in-circuit programming. A
method is then shown by which the AT89C51 microcontroller in the application can
be reprogrammed remotely, over a commercial telephone line. The circuitry described
in this application note supports five volt programming only, requiring the use of an
AT89C51-XX-5. The standard AT89C51 requires 12 volts for programming. The
software for this application may be obtained by downloading from Atmel’s.
General Considerations
EA/VPP must be held high during programming. In applications which do not
utilize external program memory, this pin may be permanently strapped to VCC.
Applications utilizing external program memory require that this pin be held low
during normal operation.
RST must be held active during programming. A means must be provided for
overriding the application reset circuit, which typically asserts RST only briefly after
power is applied.
PSEN must be held low during programming, but must not be driven during
normal operation.
ALE/PROG is pulsed low during programming, but must not be driven during
normal operation.
During programming, AT89C51 I/O ports are used for the application of mode
select, addresses and data, possibly requiring that the controller be isolated from the
application circuitry. How this is done is application dependent and will be addressed
here only in general terms.
Port Used for Input
During programming, the controller must be isolated from signals sourced by the
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