SCHMITT Pin {, Enable}

Function
Configures the internal Schmitt trigger for Pin on the SX48 or SX52. This command does not apply to the SX18, SX20, or SX28 (use the ST_B and ST_C registers).

• Pin  is any SX48/52 I/O pin (RB.0 .. RB.7, RC.0 .. RC.7, RD.0 .. RD.7, RE.0 .. RE.7).
• Enable is a constant, 0 or 1, that enables (1) or disables (0) the Schmitt trigger of Pin. When not specified, Enable defaults to 1.

Explanation
Every I/O pin in port B through port E can be set to normal or Schmitt-Trigger input. This can be configured by writing to the appropriate Schmitt-Trigger register (ST_B, ST_C, ST_D and ST_E). The I/O pins are set to normal input mode by default. Schmitt-Trigger mode can be activated for all pins, regardless of pin direction but really matter only when the associated pin is set to input mode. By configuring Schmitt-Trigger mode on input pins, the SX chip can be less sensitive to minor noise on the input pins. The figure below details the characteristics of Schmitt-Trigger inputs.

Schmitt Trigger Characteristics

Schmitt-Trigger inputs are conditioned with a large area of hysteresis. The threshold for a logic 0 is 15% of Vdd and the threshold for a logic 1 is 85% of Vdd. The input pin defaults to an unknown state until the initial voltage crosses a logic 0 or logic 1 boundary. A voltage must cross above 85% of Vdd to be interpreted as a logic 1 and must cross below 15% of Vdd to be interpreted as a logic 0. If the voltage transitions somewhere between the two thresholds, the interpreted logic state remains the same as the previous state.

Start:
  SCHMITT RB.0, 1                               ' enable Schmitt trigger
  SCHMITT RB.1                                  ' enable Schmitt trigger
  SCHMITT RB.2, 0                               ' disable Schmitt trigger

Related instructions: CMOS, TTL, and PULLUP