TC1796
System Units (Vol. 1 of 2)
Reset and Boot Operation
User’s Manual
4-1
V2.0, 2007-07
Reset, V2.0
4
Reset and Boot Operation
This chapter describes the conditions under which the TC1796 will be reset, the reset
and boot operations, and the available boot options.
4.1
Reset and Boot Overview
When the TC1796 device is first powered up, several boot parameters such as the start
location of the code have to be defined to enable proper start operation of the device. To
accommodate this, the device has a separate Power-On Reset (PORST) pin and a
number of configuration pins which are sampled during the power-on reset sequence or
the hardware reset. At the end of this sequence, the sampled values are latched, and
can not be modified until the next power-on reset. This guarantees stable conditions
during the normal operation of the device.
To reset the device while it is operating, two options exist. For reset causes coming from
the external world, a reset input pin, HDRST, is provided. If software detects conditions
that require to reset the device, it can perform a software reset through writing to a
special register, the Reset Request (RST_REQ) register.
The Watchdog Timer (WDT) module is also capable of resetting the device if it detects
a malfunction in the system. If the WDT is not serviced correctly and/or not serviced in
time, it first generates an NMI request to the CPU (this allows the CPU to gather debug
information), and then resets the device after a predefined time-out period.
After a reset has been executed, the Reset Status (RST_SR) register provides
information on the type of the last reset and the selected boot configuration.
The external reset pin, HDRST, has a double-function. It serves as a reset input from the
external world to reset the device, and it serves as a reset output to the external world to
indicate that the device has executed a reset. For this purpose, pin HDRST is
implemented as a bi-directional open-drain pin with an internal weak pull-up device.
Please note, that any reset pulse generated on HDRST will be prolonged to reach a
minimum pulse width. This ensures proper system reset, even it is caused by a very
short pulse on HDRST coming from the system.
The boot configuration information required by the device to perform the desired start
operation after a power-up reset includes the start location for the code execution, and
the activation of special modes. This information is supplied to the chip via a number of
dedicated input pins that are sampled and latched with the hardware reset HDRST or the
power-on reset PORST. However, the software reset provides the special option to alter
these parameters to allow a different start configuration after the software reset has
finished.
Attention: To ensure safe operation, the power-on reset operation must be
completed prior to utilisation of the chip. With a completed power-on
reset operation, a sequence of initialization of the chip is executed