PIC16C62B/72A
10.2.3 RC OSCILLATOR
10.3
Reset
For timing insensitive applications, the “RC” device
option offers additional cost savings. The RC oscillator
frequency is a function of the supply voltage, the resis-
tor (REXT) and capacitor (CEXT) values, and the operat-
ing temperature. In addition to this, the oscillator
frequency will vary from unit to unit due to normal pro-
cess parameter variation. Furthermore, the difference
in lead frame capacitance between package types will
also affect the oscillation frequency, especially for low
CEXT values. The user also needs to take into account
variation due to tolerance of external R and C compo-
nents used. Figure 10-4 shows how the R/C combina-
tion is connected to the PIC16CXXX.
The PIC16CXXX differentiates between various kinds
of reset:
• Power-on Reset (POR)
• MCLR reset during normal operation
• MCLR reset during SLEEP
• WDT Reset (during normal operation)
• WDT Wake-up (during SLEEP)
• Brown-out Reset (BOR)
Some registers are not affected in any reset condition;
their status is unknown on POR and unchanged by any
other reset. Most other registers are reset to a “reset
state” on Power-on Reset (POR), on the MCLR and
WDT Reset, on MCLR reset during SLEEP, and on
Brown-out Reset (BOR). They are not affected by a
WDT Wake-up from SLEEP, which is viewed as the
resumption of normal operation. The TO and PD bits
are set or cleared depending on the reset situation, as
indicated in Table 10-4. These bits are used in software
to determine the nature of the reset. See Table 10-6 for
a full description of reset states of all registers.
FIGURE 10-4: RC OSCILLATOR MODE
VDD
Rext
Internal
OSC1
clock
Cext
VSS
PIC16CXX
A simplified block diagram of the on-chip reset circuit is
shown in Figure 10-5.
OSC2/CLKOUT
Fosc/4
The PICmicro devices have a MCLR noise filter in the
MCLR reset path. The filter will ignore small pulses.
However, a valid MCLR pulse must meet the minimum
pulse width (TmcL, Specification #30).
Recommended values: 3 kΩ ≤ Rext ≤ 100 kΩ
Cext > 20pF
No internal reset source (WDT, BOR, POR) willdrive
the MCLR pin low.
1999 Microchip Technology Inc.
Preliminary
DS35008B-page 57
MICROCHIP [ MICROCHIP ]
