C8051F300/1/2/3/4/5
84
Rev. 2.9
9.1.
Power-On Reset
During powerup, the device is held in a reset state and the RST pin is driven low until V
DD
settles above
V
RST
. An additional delay occurs before the device is released from reset; the delay decreases as the V
DD
ramp time increases (V
DD
ramp time is defined as how fast V
DD
ramps from 0 V to V
RST
). For valid ramp
times (less than 1 ms), the power-on reset delay (T
PORDelay
) is typically less than 0.3 ms.
Note: The maximum V
DD
ramp time is 1 ms; slower ramp times may cause the device to be
released from reset before V
DD
reaches the VRST level.
On exit from a power-on reset, the PORSF flag (RSTSRC.1) is set by hardware to logic 1. When PORSF is
set, 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 powerup was the cause of reset. The content of internal data mem-
ory should be assumed to be undefined after a power-on reset. The V
DD
monitor is disabled following a
power-on reset.
Figure 9.2. Power-On and V
DD
Monitor Reset Timing
9.2.
Power-Fail Reset/V
DD
Monitor
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 hold the CIP-51 in a reset state (see Figure 9.2). When V
DD
returns
to a level above V
RST
, the CIP-51 will be released from the reset state. 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 reads ‘1’, the data may no longer be valid. The V
DD
monitor is disabled after power-on resets; however its defined state (enabled/disabled) is not altered by
any other reset source. For example, if the V
DD
monitor is enabled and a software reset is performed, the
V
DD
monitor will still be enabled after the reset. The V
DD
monitor is enabled by writing a ‘1’ to the PORSF
Power-On
Reset
VDD
Monitor
Reset
/RST
t
volt
s
1.0
2.0
Logic HIGH
Logic LOW
T
PORDelay
V
D
D
2.70
2.55
V
RST
VDD
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