COP840CJ Datasheet, PDF (13/36 Pages) National Semiconductor (TI) – 8-Bit Microcontrollers with Multi-Input Wake-Up and Brown Out Detector

Reset (Continued)
BROWN OUT RESET
The on-board Brown Out detection circuit resets the device
when the operating voltage (V_CC) is lower than the Brown
Out voltage. The device is held in reset when V_CC stays be-
low the Brown Out voltage. The device will remain in RESET
as long as V_CCis below the Brown Out Voltage. The device
will resume execution if V_CC rises above the Brown Out Volt-
age. If a two pin crystal/resonator clock option is selected,
the Brown Out reset will trigger a 256 tc delay. This delay al-
lows the oscillator to stabilize before the device exits the re-
set state. The delay is not used if the clock option is either
R/C or external clock. The contents of data registers and
RAM are unknown following a Brown Out reset. The external
reset takes priority over Brown Out Reset and will deactivate
the 256 tc cycles delay if in progress. The Brown Out reset
takes priority over the WATCHDOG reset.
The following actions occur as a result of Brown Out reset:
Port L:
TRI-STATE
Port G:
TRI-STATE
Port D
HIGH
PC:
CLEARED
RAM Contents:
RANDOM
B, X, SP
UNKNOWN
PSW, CNTRL1, CNTRL2 CLEARED
and WDREG Registers:
Multi-Input Wake-up
CLEARED
Registers (WKEDG,
WKEN):
(WKPND):
UNKNOWN
Data and Configuration
CLEARED
Registers for L and G:
WATCHDOG Timer:
Prescaler/Counter each
loaded with FF
Timer T1 and
Accumulator:
Unknown data after
coming
out of the HALT (through
Brown Out Reset) with any
Clock option
Note: The Development system will detect the BROWN OUT RESET exter-
nally and will force the RESET pin low. The Development System does
not emulate the 256 tc delay.
Brown Out Detection
An on-board detection circuit monitors the operating voltage
(V_CC) and compares it with the minimum operating voltage
specified. The Brown Out circuit is designed to reset the de-
vice if the operating voltage is below the Brown Out voltage
(between 1.8V–4.2V at −40C to +85C). The Minimum oper-
ating voltage for the device is 2.5V with Brown Out disabled,
but with Brown Out enabled the device is guaranteed to op-
erate properly down to minimum Brown Out voltage (Max
frequency 4 MHz). For temperature range of 0˚C–70˚C the
Brown Out voltage is expected to be between 1.9V and 3.9V.
The circuit can be enabled or disabled by Brown Out mask
option. If the device is intended to operate at lower V_CC
(lower than Brown Out voltage VBO max), the Brown Out cir-
cuit should be disabled by the mask option.
The Brown Out circuit may be used as a power-up reset pro-
vided the power supply rise time is slower than 50 µs
(0V–6.0V). Brown Out should not be used in frequencies
over 4 MHz.
Note: Brown Out Circuit is active in HALT mode (with the Brown Out mask
option selected).
Oscillator Circuits
EXTERNAL OSCILLATOR
By selecting the external oscillator option, the CKI pin can be
driven by an external clock signal provided it meets the
specified duty cycle, rise and fall times, and input levels. The
G7/CKO is available as a general purpose input G7 and/or
Halt control.
CRYSTAL OSCILLATOR
By selecting the crystal oscillator option, the G7/CKO pin is
connected as a clock output, CKI and G7/CKO can be con-
nected to make a crystal controlled oscillator. Table 1 shows
the clock frequency for different component values. See Fig-
ure 4 for the connections.
R/C OSCILLATOR
By selecting the R/C oscillator option, connecting a resistor
from the CKI pin to V_CC makes a R/C oscillator. The capaci-
tor is on-chip. The G7/CKO pin is available as a general pur-
pose input G7 and/or Halt control. Adding an external ca-
pacitor will jeopardize the clock frequency tolerance and
increase EMI emissions.
Table 2 shows the clock frequency for the different resistor
values. The capacitor is on-chip. See Figure 4 for the
connections.
DS012851-15
FIGURE 4. Clock Oscillator Configurations
13
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