iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC33/36
the actual oscillator frequency. Finally, disable In-
ternal Oscillator Test Mode in the Test Mode pane
of the Settings tab in the GUI or reset the Enable
Clock Test Mode bit (adctest) in the RB_TEST1
register and then store the updated MAIN_CLOCK
register value to EEPROM using a Write All com-
mand via one of the serial ports or the Write
EEPROM button in the GUI.
Tuning the Internal Oscillator
The iC-TW8’s internal oscillator has a nominal fre-
quency of 20 MHz with 5V supplies and 16 MHz
with 3.3V supplies. However, manufacturing toler-
ances and changes in temperature can cause large
variations in the actual internal oscillator frequency
of any given device.
The clock tuning value in the MAIN_CLOCK.freq
variable allows tuning the frequency of the internal
oscillator. In addition, the clock divider value in the
MAIN_CLOCK.div variable allows dividing the
oscillator frequency by 1, 2, or 4. These two param-
eters allow setting the internal oscillator of any spe-
cific device to the desired frequency. The
MAIN_CLOCK register can be accessed using the
serial ports (see Programmer’s Reference) or via the
Settings tab in the GUI.
Typical internal oscillator frequency of the iC-TW8
with 5V supplies and a clock divider of 1 at room
temperature (20°C) is shown below. Other clock
divider values produce appropriately scaled-down
frequencies.
Typical Internal Oscillator Frequency
Clock
Tuning
Value
0
FRAME Output
Frequency
[MHz]
10.6
fcore
Frequency
[MHz]
21.2
To observe the internal oscillator output, enable In-
ternal Oscillator Test Mode in the Test Mode pane
of the Settings tab in the GUI or set the Enable
Clock Test Mode bit (adctest) in the RB_TEST1
register in the RB Block of internal memory using
serial port commands (see Programmer’s Refer-
ence). This forces fcore/2 onto the FRAME output
(pin 8), as shown below, where it can be observed
using an oscilloscope or a frequency counter.
1
2
3
4
5
6
7
8
11.0
11.5
11.9
12.5
13.0
13.6
14.3
15.1
15.8
16.8
17.7
18.9
20.2
21.6
23.3
22.0
23.0
23.8
25.0
26.0
27.2
28.6
30.2
31.6
33.6
35.4
37.8
40.4
43.2
46.6
9
10
11
12
13
14
15
MAIN_CLOCK.div
Internal
Oscillator
Clock
Divider
fcore
Note that the nominal oscillator frequency is
achieved with a clock tuning value of 0 and that the
range of the clock tuning value allows setting the
internal oscillator frequency above the maximum
allowed for proper operation of the device (shown in
red). Setting the internal oscillator frequency higher
than the specified maximum may result in undefined
operation.
÷2
Clock
Tuning
Value
FRAME
MAIN_CLOCK.freq
In general, set the internal oscillator frequency as
close to—but not greater than—the maximum fre-
quency for the supply voltage used (32 MHz at 5V,
24MHz at 3.3V). The temperature coefficient of the
internal oscillator is negative, so oscillator frequen-
cy decreases with rising temperature and increases
with decreasing temperature. In applications where
low temperature operation is expected, the room
Figure 36: Internal Oscillator Tuning
First, set the clock divider to the desired value, usu-
ally 1 (MAIN_CLOCK.div = 3), and the clock tun-
ing value to its lowest value (MAIN_CLOCK.freq =
0). Increment the clock tuning value until the de-
sired oscillator frequency is obtained, remembering
that the output frequency on the FRAME pin is half
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