Echelon FTXL Hardware - Manual (Page 34)

26 FTXL Transceiver Hardware Interface
• Program recovery
If an application experiences unexpected behavior because of address or
data corruption, a reset can recover.
• V
DD
power down
Reset ensures proper shutdown.
• Memory maintenance
Reset helps protect the EEPROM from major corruption.
The FTXL Transceiver has four mechanisms to initiate a reset:
• The RESET~ pin is asserted (pulled low) and then deasserted (returned
high).
• A software reset command from the parallel interface driver within the
FTXL LonTalk protocol stack.
• A watchdog timeout occurs during application execution (the timeout
period is approximately 840 ms when running at 10 MHz and
approximately 210 ms at 40 MHz; this figure scales inversely with clock
frequency).
• The LVI circuit detects a drop in the power supply below a set level.
During any of the reset functions, when the RESET~ pin is asserted (in the low
state), the FTXL Transceiver enters the following states:
• The oscillator continues to run
• All processor functions stop
• The SERVICE~ pin goes to high impedance
• The I/O pins go to high impedance
When the RESET~ pin is released back to a high state, the FTXL Transceiver
begins its initialization procedure.
Power-Up Sequence
During power up sequences, the RESET~ pin should be held low until the power
supply is stable, to prevent start-up malfunctioning. Likewise, when powering
down, the FTXL Transceiver RESET~ pin should be asserted before the power
supply goes below the minimum operating voltage of the FTXL Transceiver.
Important: If proper reset recovery circuitry is not used, the FTXL Transceiver
can go applicationless. The applicationless state occurs when the checksum
error-verification routine detects a corruption in memory; this corruption can be
falsely detected because of an improper reset sequence or noise on the power
supply. In general, the applicationless state is an unrecoverable state, so be sure
that your device’s reset circuitry is correct.
The total capacitance directly connected to the RESET~ pin, including stray and
external device input capacitance, must not exceed 1000 pF. This limit ensures
that the FTXL Transceiver can successfully output a reset down to below 0.8 V.
The 100 pF minimum capacitance is required for noise immunity.
Because the FTXL LonTalk protocol stack internally handles most reset events,
you do not generally need an external Reset button or switch connected to the
RESET~ pin.