4.3 Master Clear (MCLR) Pin

The MCLR pin provides two specific device functions: Device Reset and Device Programming and Debugging. If programming and debugging are not required in the end application, a direct connection to VDD may be all that is required. The addition of other components, to help increase the application’s resistance to spurious Resets from voltage sags, may be beneficial. A typical configuration is shown in Figure 4-1. Other circuit designs may be implemented, depending on the application’s requirements.

During programming and debugging, the resistance and capacitance that can be added to the pin must be considered. Device programmers and debuggers drive the MCLR pin. Consequently, specific voltage levels (VIH and VIL) and fast signal transitions must not be adversely affected. Therefore, specific values of R1 and C1 will need to be adjusted based on the application and PCB requirements. For example, it is recommended that the capacitor, C1, be isolated from the MCLR pin during programming and debugging operations by using a jumper (Figure 4-2). The jumper is replaced for normal run-time operations.

Any components associated with the MCLR pin need to be placed within 0.25 inch (6 mm) of the pin.

Figure 4-2. Example of MCLR Pin Connections
  1. R1 ≤ 10 kΩ is recommended. A suggested starting value is 10 kΩ. Ensure that the MCLR pin VIH and VIL specifications are met.
  2. R2 ≤ 470Ω will limit any current flowing into MCLR from the extended capacitor, C1, in the event of MCLR pin breakdown, due to Electrostatic Discharge (ESD) or Electrical Overstress (EOS). Ensure that the MCLR pin VIH and VIL specifications are met.