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TC1796
System Units (Vol. 1 of 2)
Reset and Boot Operation
User’s Manual
4-8
V2.0, 2007-07
Reset, V2.0
The HDRST (output) pin is held low for 1024
f
SYS
clock cycles by the reset circuitry until
its internal reset sequence is terminated.
When the sequence is terminated, the reset circuitry then releases HDRST (that is, it
does not actively drive HDRST anymore, so that the weak pull-up can try to drive the pin
high). It then begins monitoring the level of the pin. If HDRST is still low (indicating that
it is still being driven low externally), the reset circuitry holds the chip in hardware reset
until a high level is detected. The hardware reset sequence is then terminated and flag
RST_SR.HDRST is set.
The PLL is not affected by an external hardware reset and continues to operate.
In order to safely recognize a valid hardware reset, HDRST must be active for at least
four
f
CPU
clock cycles.
HDRST is equipped with a noise-suppression filter which suppresses glitches below
10 ns pulse width. HDRST pulses with a width above 100 ns are safely recognized as a
valid signal. The noise-suppression filter is switched-off when pin BYPASS = 1.
4.2.3
Software Reset
A software reset is invoked by writing the appropriate bits in the reset request register
RST_REQ. Unlike the other reset types, the software reset can include/exclude
optionally the system timer reset and the external reset output HDRST generation from
becoming active. To exclude one of these two system functions from software reset, the
corresponding bits in RST_REQ (RRSTM or RREXT) must be set to 0. Additionally, a
software reset can be executed with a programmable software boot configuration value
(bit field RST_REQ.SWCFG) instead of the last hardware boot configuration value (as
latched in bit field RST_SR.HWCFG).
To perform a software reset, the reset request register RST_REQ must be written.
However, RST_REQ is Endinit-protected to avoid triggering of an unintentional software
reset. Bit WDT_CON0.ENDINIT must be cleared via the password-protected access
scheme. When this is done, a write access to RST_REQ can then be performed.
After the write access to RST_REQ, the entering into the software reset takes some time
in which instructions can be still executed. Therefore, it is recommended not to execute
important instructions anymore after the instruction that writes to RST_REQ (e.g
entering a endless loop).
4.2.4
Watchdog Timer Reset
A Watchdog Timer overflow or access error occurs only in response to severe and/or
unknown malfunctions of the TC1796, either caused by software or hardware errors.
Therefore, a Watchdog Timer reset occurs when an overflow of the Watchdog Timer
takes place.
Before the Watchdog Timer generates its reset, it first indicates a Non-Maskable
Interrupt (NMI) at the beginning of a prewarning phase and enters a time-out mode. The