8-bit microcontroller with accelerated two-clock 80C51 core
8.15.3 Total Power-down mode
This is the same as Power-down mode except that the brownout detection circuitry and
the voltage comparators are also disabled to conserve additional power. The internal RC
oscillator is disabled unless both the RC oscillator has been selected as the system clock
and the RTC is enabled. If the internal RC oscillator is used to clock the RTC during
power-down, there will be high power consumption. Please use an external low frequency
clock to achieve low power with the RTC running during power-down.
The P1.5/RST pin can function as either a LOW-active reset input or as a digital input,
P1.5. The Reset Pin Enable (RPE) bit in UCFG1, when set to logic 1, enables the external
reset input function on P1.5. When cleared, P1.5 may be used as an input pin.
Remark: During a power-up sequence, the RPE selection is overridden and this pin will
always functions as a reset input. An external circuit connected to this pin should not
hold this pin LOW during a power-on sequence as this will keep the device in reset.
After power-up this input will function either as an external reset input or as a digital input
as defined by the RPE bit. Only a power-up reset will temporarily override the selection
defined by RPE bit. Other sources of reset will not override the RPE bit. When this pin
functions as a reset input, an internal pull-up resistance is connected (see Table 11 “Static
Reset can be triggered from the following sources:
• External reset pin (during power-up or if user configured via UCFG1).
• Power-on detect.
• Brownout detect.
• Watchdog timer.
• Software reset.
• UART break character detect reset.
For every reset source, there is a flag in the reset register, RSTSRC. The user can read
this register to determine the most recent reset source. These flag bits can be cleared in
software by writing a logic 0 to the corresponding bit. More than one flag bit may be set:
• During a power-on reset, both POF and BOF are set but the other flag bits are
• For any other reset, previously set flag bits that have not been cleared will remain set.
8.16.1 Reset vector
Following reset, the P89LPC933/934/935/936 will fetch instructions from either address
0000H or the boot address. The boot address is formed by using the boot vector as the
high byte of the address and the low byte of the address = 00H.
The boot address will be used if a UART break reset occurs, or the non-volatile boot
status bit (BOOTSTAT.0) = 1, or the device is forced into ISP mode during power-on (see
P89LPC933/934/935/936 User manual). Otherwise, instructions will be fetched from
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 8 — 12 January 2011
© NXP B.V. 2011. All rights reserved.
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