C8051F124-GQR Datasheet, PDF (178/350 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
13.1. Power-on Reset
The C8051F120/1/2/3/4/5/6/7 family incorporates a power supply monitor that holds the MCU in the reset
state until V_DD rises above the V_RST level during power-up. See Figure 13.2 for timing diagram, and refer
to Table 13.1 for the Electrical Characteristics of the power supply monitor circuit. The RST pin is asserted
low until the end of the 100 ms V_DD Monitor timeout in order to allow the V_DD supply to stabilize. The V_DD
Monitor reset is enabled and disabled using the external V_DD monitor enable pin (MONEN). When the V_DD
Monitor is enabled, it is selected as a reset source using the PORSF bit. If the RSTSRC register is written
by firmware, PORSF (RSTSRC.1) must be written to ‘1’ for the V_DD Monitor to be effective.
On exit from a power-on reset, the PORSF flag (RSTSRC.1) is set by hardware to logic 1. All of the other
reset flags in the RSTSRC Register are indeterminate. PORSF is cleared by all other resets. Since all
resets cause program execution to begin at the same location (0x0000) software can read the PORSF flag
to determine if a power-up was the cause of reset. The contents of internal data memory should be
assumed to be undefined after a power-on reset.
2.70
2.55
2.0
1.0
V_RST
Logic HIGH
/RST
Logic LOW
100ms
t
100ms
Power-On Reset
VDD Monitor Reset
Figure 13.2. Reset Timing
13.2. Power-fail Reset
When a power-down transition or power irregularity causes V_DD to drop below V_RST, the power supply
monitor will drive the RST pin low and return the CIP-51 to the reset state. When V_DD returns to a level
above VRST, the CIP-51 will leave the reset state in the same manner as that for the power-on reset (see
Figure 13.2). Note that even though internal data memory contents are not altered by the power-fail reset,
it is impossible to determine if V_DD dropped below the level required for data retention. If the PORSF flag is
set to logic 1, the data may no longer be valid.
178
Rev. 1.4