ft7521 Fairchild Semiconductor, ft7521 Datasheet - Page 3

• Manufacturer Part Number: ft7521
• Description: Reset Timer With Fixed Delay Time And Reset Pulse
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FT7521 • Rev. 1.0.1
Functional Description
Device default operation time N is 7.5s. If the DSR pin is
pulled HIGH prior to V
Mode and the reset output, /RST1, is immediately pulled
LOW for factory testing. The DSR pin MUST be forced
to GND during normal operation. The DSR pin should
never be driven HIGH or left to FLOAT during normal
operation. The DSR PIN state should never be changed
during device operation; it must be biased prior to
supplying the V
DSR=VCC Test Mode, the /SR0 must be HIGH when
the DSR pin is moved from LOW to HIGH to enter 0-
Second Factory-Test Mode. To return to the standard
7.5-second reset time, the same procedure must be
followed with DSR=GND. The DSR pin should never be
allowed to change state while the /SR0 pin is LOW.
Operation Modes
A low input signal on /SR0 starts the oscillator. There
are two scenarios for counting, short duration and long
duration. In the short-duration scenario, output /RST1 is
not affected. In the long-duration scenario, the output
/RST1 goes LOW after /SR0 has been held LOW for
7.5s. The /RST1 output returns to its original HIGH
CC
supply. If there is a need to use the
CC
ramp, the FT7521 enters Test
Figure 3.
Reset Timing Waveforms
3
state 400ms after time t
the state of /SR0. The /RST1 output is an open-drain
driver. When the count time exceeds time 7.5s, the
/RST1 output pulls LOW.
Short Duration (t
When the /SR0 input goes LOW, the internal timer starts
counting. If the /SR0 input goes HIGH before 7.5s has
elapsed. The timer stops counting and resets; no
changes occur on the outputs.
Long Duration (t
When the /SR0 input goes LOW, the internal timer starts
counting. If the /SR0 input stays LOW for at least 7.5s, the
RST output is enabled and pulled LOW. The output RST
is held LOW for tREC time, 400ms, as soon as the reset
time of 7.5s is met, regardless of the state of the /SR0
pin. When the /SR0 input has returned HIGH and the
tREC time has expired, the internal timer resets and
awaits the next RESET event.
0-Second Test Mode
/RST1 goes LOW immediately after /SR0 goes LOW.
W
W
> 7.5s)
< 7.5s)
REC
has expired, regardless of
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