ML51Dec. 05, 2018 Page 159 of 401 Rev 1.00ML51 SERIES TECHNICAL REFERENCE MANUAL9.3 Input-Only ModeInput-only mode provides true high-impedance input path. Although a quasi-bidirectional mode I/O canalso be an input pin, but it requires relative strong input source. Input-only mode also benefits to powerconsumption reduction for logic 0 input always consumes current from VDD if in quasi-bidirectionalmode. User needs to take care that an input-only mode pin should be given with a determined voltagelevel by external devices or resistors. A floating pin will induce leakage current especially in Power-down mode.Port PinInputFigure 9.3-1 Input-Only Mode Structure9.4 Open-Drain ModeThe open-drain mode turns off all pull-high transistors and only drives the pull-low of the port pin whenthe port latch is given by logic 0. If the port latch is logic 1, it behaves as if in input-only mode. To beused as an output pin generally as I2C lines, an open-drain pin should add an external pull-high,typically a resistor tied to VDD. User needs to take care that an open-drain pin with its port latch aslogic 1 should be given with a determined voltage level by external devices or resistors. A floating pinwill induce leakage current especially in Power-down mode.Port PinInputPort Latch NFigure 9.4-1 Open-Drain Mode Structure9.5 Read-Modify-Write InstructionsInstructions that read a byte from SFR or internal RAM, modify it, and rewrite it back, are called “Read-Modify-Write” instructions. When the destination is an I/O port or a port bit, these instructions read theinternal output latch rather than the external pin state. This kind of instructions read the port SFRvalue, modify it and write back to the port SFR. All “Read-Modify-Write” instructions are listed asfollows.Instruction DescriptionANL Logical AND. (ANL direct, A and ANL direct, #data)
