Clocking
September 5, 2002
Document #: 38-12010 CY Rev. ** CMS Rev. 3.20
35
7.0
Clocking
7.1
Oscillator Options
7.1.1
Internal Main Oscillator
The internal main oscillator outputs two frequencies, 48
MHz and 24 MHz. In the absence of a high-precision
input source from the external oscillator, the accuracy of
this circuit is +/- 2.5% (between 0oC and +85oC). No
external components are required to achieve this level of
accuracy. The Internal Main Oscillator Trim Register
(IMO_TR) is used to calibrate this oscillator into specified
tolerance. Factory-programmed trim values are available
for 5.0V and 3.3V operation. The 5.0V value is loaded in
the IMO_TR register upon reset. This register must be
adjusted when the operating voltage is outside the range
for which factory calibration was set. The factory-pro-
grammed trim value is selected using the Table Read
Supervisor Call, and is documented in 11.8.
There is an option to phase lock this oscillator to the
External Crystal Oscillator. The choice of crystal and its
inherent accuracy will determine the overall accuracy of
the oscillator. The External Crystal Oscillator must be
stable prior to locking the frequency of the Internal Main
Oscillator to this reference source.
Internal Main Oscillator Trim Register (IMO_TR, Address = Bank 1, E8h)
7.1.2
Internal Low Speed Oscillator
An internal low speed oscillator of nominally 32 kHz is
available to generate sleep wake-up interrupts and
Watchdog resets if the user does not want to attach a
32.768 kHz watch crystal. This oscillator can also be
used as a clocking source for the digital PSoC blocks.
The oscillator operates in two different modes. A trim
value is written to the Internal Low Speed Oscillator Trim
Register (ILO_TR), shown below, upon reset. See sec-
tion 13.0 for accuracy information. When the IC is put
into sleep mode this oscillator drops into an ultra low cur-
rent state and the accuracy is reduced.
This register sets the adjustment for the Internal Low
Speed Oscillator. The value placed in this register is
based on factory testing. It is recommended that the user
not alter this value.
Table 35:
Internal Main Oscillator Trim Register
Bit #
7
6
5
4
3
2
1
0
POR
FS1
FS1
FS1
FS1
FS1
FS1
FS1
FS1
Read/Write
W
W
W
W
W
W
W
W
Bit Name
IMOTrim
[7]
IMO Trim
[6]
IMO Trim
[5]
IMO Trim
[4]
IMO Trim
[3]
IMO Trim
[2]
IMO Trim
[1]
IMO Trim
[0]
Bit [7:0]
: IMO Trim [7:0] Data value stored will alter the trimmed frequency of the Internal Main Oscillator. A larger
value in this register will increase the speed of the Internal Main Oscillator
1.
FS = Factory set trim value
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