Maxim Integrated Ultra-High-Speed Flash Microcontroller - Manual (Page 120)

10-7
Transition current is available to help move the port pin from logic 0 to logic 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 logic 0 to logic 1. Writing
logic 1 where a 1 was already in place does not change the strength of the pullup. This transition current is applied for two oscillator
cycles. The absolute current is not guaranteed, but is approximately 2 mA at 5V.
When serving as an I/O port, the drive varies as follows: for logic 0, the port invokes a strong pulldown; for logic 1, the port invokes a
strong pullup for two oscillator cycles to assist with the logic transition. Then the port reverts to a weak pullup. This weak pullup is main-
tained until the port transitions from logic 1 to logic 0. External circuits can overdrive the weak pullup. This allows the logic 1 output
state to serve as the input state as well.
Substantial DC current is available in both the high and low levels. However, the power dissipation limitations make it inadvisable to heav-
ily load multiple pins. In general, sink and source currents should not exceed 10mA total per port (8 bits) and 25mA total per package.
Input Functions
The input state of the I/O ports is the same as that of the output logic 1. That is, the pin is pulled weakly to logic 1. This logic 1 state is
easily overcome by external components. Thus, after software writes a 1 to the port pin, the port is configured for input. When the port
is read by software, the state of the pin is read. The only exception is the read-modify-write instructions, discussed earlier. If the exter-
nal circuit is driving logic 1, then the pin is logic 1. If the external circuit is driving logic 0, then it overcomes the internal pullup. Thus,
the pin is the same as the driven logic state. Note that the port latch is not altered by a read operation. Therefore, if logic 0 is driven
onto a port pin from an external source, then removed, the pin reverts to the weak pullup, as determined by the internal latch.
Ultra-High-Speed Flash
Microcontroller User’s Guide
Maxim Integrated