Echelon FT 3150 Smart Transceiver - Manual (Page 25)

FT 3120 / FT 3150 Smart Transceiver Data Book 19
Additional Functions
Figure 2.9 Test Point Levels for CLK1 Duty Cycle Measurements
The FT 3120 Smart Transceiver was designed to run at frequencies up to 40MHz using an external clock oscillator.
External oscillators generally take several milliseconds to stabilize after power-up. The FT 3120 Smart Transceiver
operating at 40MHz must be held in reset until the externally-generated CLK input is stable, so an external power-
on-reset-pulse stretching LVI chip/circuit is required. Check the specification of the oscillator vendor for more
information about startup stabilization times.
Additional Functions
Reset Function
The reset function is a critical operation in any embedded microcontroller. In the case of theFT 3120 and FT 3150
Smart Transceivers, the reset function plays a key role in the following conditions:
• Initial V
DD
power up (ensures proper initialization of the FT Smart Transceiver).
• V
DD
power fluctuations (manages proper recovery of FT Smart Transceiver after V
DD
stabilizes).
• Program recovery (if an application gets lost due to corruption of address or data, an external reset can be
used for recovery or the watchdog timer could timeout, causing a watchdog reset).
• V
DD
power down (ensures proper shut down).
• Helps protect the EEPROM from major corruption.
The FT Smart Transceivers have four mechanisms to initiate a reset:
• RESET pin is pulled low and then returned high.
• Watchdog timeout occurs during application execution (the timeout period is 210ms at 40MHz; this figure
scales inversely with clock frequency).
Figure 2.8 Smart Transceiver Clock Generator Circuit
EXTERNAL
CRYSTAL
R
f
C C
R
CLK 2 CLK1
PW
LOW
V
DD
/2
PW
HIGH
V
DD
/2 V
DD
/2