DS80C390_00 Datasheet, PDF (105/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

DS80C390 High-Speed Microcontroller User’s Guide Supplement
logic values will be output. DC drive capability is provided in the electrical specifications. Note that the
DC current available from an I/O port pin is a function of the permissible voltage drop. Transition current
is available to help move the port pin from a 0 to a 1. Since the logic 0 driver is strong, no additional
drive current is needed in the 1 to 0 direction. The transition current is applied when the port latch is
changed from a logic 0 to a logic 1. Simply writing a logic 1 where a 1 was already in place does not
change the strength of the pull-up. This transition current is applied for a one half of a machine cycle. The
absolute current is not guaranteed, but is approximately 2 mA at 5V.
When serving as an I/O port, the drive will vary as follows. For a logic 0, the port will invoke a strong
pull-down. For a logic 1, the port will invoke a strong pull-up for two oscillator cycles to assist with the
logic transition. Then, the port will revert to a weak pull-up. This weak pull-up will be maintained until
the port transitions from a 1 to a 0. The weak pull-up can be overdriven by external circuits. This allows
the output 1 state to serve as the input state as well.
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