Q
The PSEN pin is always an output on an 8051. How then can it be used as an
input to enter serial
download mode? What pulldown resistor should be used to be sure to enter serial
download
mode? Is the standard functionality of the PSEN pin affected?
A
On an 8051 the PSEN pin is always an output. It is used to access external code
memory. On the
Microcontroller the PSEN pin is used to access external code memory and is used
as an input to
determine whether to run user code or run the on chip serial downloader.
The PSEN pin is normally configured as an output. When the RESET pin is pulled
high, the
PSEN pin becomes a digital input and the voltage at the PSEN pin is sampled
every machine
cycle. Once the RESET pin is released (low) the last sampled logic level of the
PSEN pin is used
to decide whether to run from user code or whether to run the on chip serial
downloader.
To ensure that the PSEN pin is interpreted as a logic low it should be pulled
low through a 1k
resistor to ground. To ensure that the PSEN pin is interpreted as a logic high
it can be left float as
there is an internal pullup resistor.
Because the functionality of the PSEN pin on the ADUC8xx Microcontroller only
differs from that of an
8051 when RESET is pulled high (in this condition the PSEN pin on an 8051 is
useless) the
functionality of the PSEN pin on the ADUC8xx Microcontroller is unaffected by
its additional functionality.
