TC1796
System Units (Vol. 1 of 2)
Clock System and Control
User’s Manual
3-21
V2.0, 2007-07
Clock, V2.0
3.2.3
Power-on Startup Operation
In order to support a wide range of input clock frequencies, the TC1796 requires a
generic procedure to start-up the system clock.
When the TC1796 is powered up, a low level (0) must be applied to the power-on reset
pin, PORST. While PORST is active (at low level), the device is asynchronously held in
reset and the state of the BYPASS pin controls the operation of the clock circuitry.
Therefore, BYPASS must be held at constant level during PORST active.
If the BYPASS pin is at a high level during power-on reset, Direct Drive Mode is selected
(see
). The low level at pin PORST has to be held long enough to make sure
that a stable clock is provided to the TC1796. In case of an external crystal oscillator, it
can take several ms until the oscillator has started up and is stable. If the clock input is
provided by another clock source with faster startup characteristics, the PORST low level
can be released earlier.
If the BYPASS pin is at low level during power-on reset, PLL Mode is selected (see
) and the procedure to start-up the PLL is as described in the next paragraph.
With PORST = 0, the oscillator is disconnected from the PLL. The PLL starts running at
the VCO base frequency
f
VCObase
. After deactivation of the PORST, the CPU and system
clock are provided internally with a frequency that is equal to
f
VCObase
/K (K = 16 after
reset). This means that in the TC1796, the Boot ROM code execution is started with the
VCO base frequency. When the oscillator is running properly, bit OSCR becomes set
and the user software can setup the PLL by programming its parameters as described
in
. If OSCR becomes not set, the user software has the possibility to run
an emergency program using the base frequency of the PLL divided by K. In this case,
K can be set to the minimum value, which results in the maximum possible CPU and
system frequency.
Note: See also
for further details on the power-on reset operation.