The PIC18F2420/2520/4420/4520 devices differentiate between various kinds of Reset:

  1. Power-on Reset (POR)
  2. MCLR Reset during normal operation
  3. MCLR Reset during power-managed modes
  4. Watchdog Timer (WDT) Reset (during execution)
  5. Programmable Brown-out Reset (BOR)
  6. RESET Instruction
  7. Stack Full Reset
  8. Stack Underflow Reset

This section discusses Resets generated by MCLR,POR and BOR and covers the operation of the various start-up timers. “Stack Full and Underflow Resets”. “Watchdog Timer (WDT)”.

A simplified block diagram of the On-Chip Reset Circuit is shown in fig. 


RCON Register

Device Reset events are tracked through the RCON register (Register 4-1). The lower five bits of the register indicate that a specific Reset event has occurred. In most cases, these bits can only be cleared by the event and must be set by the application after the event. The state of these flag bits, taken together, can be read to indicate the type of Reset that just occurred. This is described in more detail in Section “Reset State of Registers”.

The RCON register also has control bits for setting interrupt priority (IPEN) and software control of the BOR (SBOREN). BOR is covered in Section 4.4 “Brown-out Reset (BOR)”.

RCON Structure

Bit Explanation
bit 7IPEN: Interrupt Priority Enable bit
1 = Enable priority levels on interrupts
0 = Disable priority levels on interrupts (PIC16CXXX Compatibility mode)
bit 6SBOREN: BOR Software Enable bit(1)
If BOREN1:BOREN0 = 01:
1 = BOR is enabled
0 = BOR is disabled
If BOREN1:BOREN0 = 00, 10 or 11:
Bit is disabled and read as ‘0’.
bit 5Unimplemented: Read as ‘0’
bit 4RI: RESET Instruction Flag bit
1 = The RESET instruction was not executed (set by firmware only)
0 = The RESET instruction was executed causing a device Reset (must be set in software after a
Brown-out Reset occurs)
bit 3TO: Watchdog Time-out Flag bit
1 = Set by power-up, CLRWDT instruction or SLEEP instruction
0 = A WDT time-out occurred
bit 2PD: Power-Down Detection Flag bit
1 = Set by power-up or by the CLRWDT instruction
0 = Set by execution of the SLEEP instruction
bit 1POR: Power-on Reset Status bit
1 = A Power-on Reset has not occurred (set by firmware only)
0 = A Power-on Reset occurred (must be set in software after a Power-on Reset occurs)
bit 0BOR: Brown-out Reset Status bit
1 = A Brown-out Reset has not occurred (set by firmware only)
0 = A Brown-out Reset occurred (must be set in software after a Brown-out Reset occurs)
Master Clear (MCLR)

The MCLR pin provides a method for triggering an external Reset of the device. A Reset is generated by holding the pin low. These devices have a noise filter in the MCLR Reset path which detects and ignores small pulses. The MCLR pin is not driven low by any internal Resets, including the WDT. In PIC18F2420/2520/4420/4520 devices, the MCLR input can be disabled with the MCLRE Configuration bit. When MCLR is disabled, the pin becomes a digital input.

Power-on Reset (POR)

A Power-on Reset pulse is generated on-chip whenever VDD rises above a certain threshold. This allows the device to start in the initialized state when VDD is adequate for operation. To take advantage of the POR circuitry, tie the MCLR pin through a resistor (1 k&Omega; to 10 k&Omegaemoticon_wink to VDD. This will eliminate external RC components usually needed to create a Power-on Reset delay. A minimum rise rate for VDD is specified (parameter D004). For a slow rise time, see Figure 4-2. When the device starts normal operation (i.e., exits the Reset condition), device operating parameters (voltage, frequency, temperature, etc.) must be met to ensure operation. If these conditions are not met, the device must be held in Reset until the operating conditions are met. POR events are captured by the POR bit (RCON<1>). The state of the bit is set to ‘0’ whenever a POR occurs; it does not change for any other Reset event. POR is not reset to ‘1’ by any hardware event. To capture multiple events, the user manually resets the bit to ‘1’ in software following any POR.

Brown-out Reset (BOR)

PIC18F2420/2520/4420/4520 devices implement a BOR circuit that provides the user with a number of configuration and power-saving options. The BOR is controlled by the BORV<1:0> and BOREN<1:0>
Configuration bits. There are a total of four BOR configurations which are summarized in Table 4-1. The BOR threshold is set by the BORV<1:0> bits. If BOR is enabled (any values of BOREN<1:0>, except ‘00’), any drop of VDD below VBOR (parameter D005) for greater than TBOR (parameter 35) will reset the device. A Reset may or may not occur if VDD falls below VBOR for less than TBOR. The chip will remain in Brown-out Reset until VDD rises above VBOR. If the Power-up Timer is enabled, it will be invoked after VDD rises above VBOR; it then will keep the chip in Reset for an additional time delay, TPWRT (parameter 33). If VDD drops below VBOR while the Power-up Timer is running, the chip will go back into a Brown-out Reset and the Power-up Timer will be initialized. Once VDD rises above VBOR, the Power-up Timer will execute the additional time delay. BOR and the Power-up Timer (PWRT) are independently configured. Enabling the Brown-out Reset does not automatically enable the PWRT.

brown out reset

Device Reset Timers

PIC18F2420/2520/4420/4520 devices incorporate three separate on-chip timers that help regulate the Power-on Reset process. Their main function is to ensure that the device clock is stable before code is executed. These timers are:

  • Power-up Timer (PWRT)
  • Oscillator Start-up Timer (OST)
  • PLL Lock Time-out

The Power-up Timer (PWRT) of PIC18F2420/2520/ 4420/4520 devices is an 11-bit counter which uses the INTRC source as the clock input. This yields an approximate time interval of 2048 x 32 μs = 65.6ms. While the PWRT is counting, the device is held in Reset.The power-up time delay depends on the INTRC clock and will vary from chip to chip due to temperature and process variation. See DC parameter 33 for details. The PWRT is enabled by clearing the PWRTEN Configuration bit.


The Oscillator Start-up Timer (OST) provides a 1024 oscillator cycle (from OSC1 input) delay after the PWRT delay is over (parameter 33). This ensures that the crystal oscillator or resonator has started and stabilized. The OST time-out is invoked only for XT, LP, HS and HSPLL modes and only on Power-on Reset, or on exit from most power-managed modes.


With the PLL enabled in its PLL mode, the time-out sequence following a Power-on Reset is slightly different from other oscillator modes. A separate timer is used to provide a fixed time-out that is sufficient for the PLL to lock to the main oscillator frequency. This PLL lock time-out (TPLL) is typically 2 ms and follows the oscillator start-up time-out.


  • Notes, Edited, Created by Prof Sujit Wagh, SKNCOE, Pune
  • WikiNote Foundation

Last modified: Tuesday, 17 September 2019, 12:31 PM