CY8C28xxx PSoC Programmable System-on-Chip TRM, Document No. 001-52594 Rev. *G
81
8. Internal Main Oscillator (IMO)
This chapter presents the Internal Main Oscillator (IMO) and its associated registers. The IMO produces clock signals of
24 MHz and 48 MHz. For a complete table of the IMO registers, refer to the
“Summary Table of the Core Registers” on
. For a quick reference of all PSoC
®
registers in address order, refer to the
Register Details chapter on page 125
.
8.1
Architectural Description
The Internal Main Oscillator (IMO) outputs two clocks: a
SYSCLK, which can be the internal 24 MHz clock or an
external clock, and a SYSCLKX2 that is always twice the
SYSCLK frequency. In the absence of a high-precision input
source from the 32.768 kHz
of the internal 24/48 MHz clocks will be ±2.5% over temper-
ature variation and two voltage ranges (3.3 V ± 0.3 V and
5.0 V ± 0.25 V). No external components are required to
achieve this level of accuracy.
There is an option to phase lock this oscillator to the Exter-
nal Crystal Oscillator (ECO). The choice of crystal and its
inherent accuracy will determine the overall accuracy of the
oscillator. The ECO must be stable prior to locking the fre-
quency of the IMO to this reference source.
The
doubler circuit, which produces SYSCLKX2,
can be disabled to save power. On CY8C28xxx PSoC
devices, lower frequency SYSCLK settings are available by
setting the slow IMO (SLIMO) bit in the CPU_SCR1 register.
With this bit set and the corresponding factory trim value
applied to the IMO_TR register, SYSCLK can be lowered to
6 MHz. This offers lower device power consumption for sys-
tems that can operate with the reduced system clock. Slow
IMO mode is discussed further in the
.
8.2
Application Description
To save power, the IMO frequency can be reduced from 24
MHz to 6 MHz using the SLIMO bit in the CPU_SCR1 regis-
ter, in conjunction with the Trim values in the IMO_TR regis-
ter. How to do this is described in the sections that follow.
8.2.1
Trimming the IMO
An 8-bit register (IMO_TR) is used to trim the IMO. Bit 0 is
the LSB and bit 7 is the MSB. The trim step size is approxi-
mately 80 kHz.
A factory trim setting is loaded into the IMO_TR register at
boot time for 5 V ± 0.25 V operation,. For operation in the
voltage ranges of 3.3 V ± 0.3 V, user code must modify the
contents of this register with values stored in Flash bank 0
as shown in
. This is done with a
Table Read command to the Supervisory ROM.
8.2.2
Engaging Slow IMO
Forcing CPU_SCR1 register bit 4 high engages the Slow
IMO feature. The IMO will immediately drop to a lower fre-
quency. Factory trim settings are stored in Flash bank 0 as
shown in
for the following voltage/fre-
quency combinations.
A Table Read command to the Supervisory ROM is per-
formed to set the IMO to the different frequencies.
Voltage
Normal IMO Frequency
Slow IMO Frequency
5.0 V ± 0.25 V
24 MHz
6 MHz
3.3 V ± 0.3 V
24 MHz
6 MHz
Содержание CY8C28 series
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