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CY8C28xxx PSoC Programmable System-on-Chip TRM, Document No. 001-52594 Rev. *G
Sleep and Watchdog
12.3.5
OSC_CR0 Register
The Oscillator Control Register 0 (OSC_CR0) is used to
configure various features of internal clock sources and
clock nets.
Bit 7: 32k Select.
By default, the 32 kHz clock source is the
Internal Low Speed Oscillator (ILO). Optionally, the 32.768
kHz External Crystal Oscillator (ECO) may be selected.
Bit 6: PLL Mode.
This is the only bit in the OSC_CR0 reg-
ister that directly influences the Phase Locked Loop (PLL).
When set, this bit enables the PLL. The EXTCLKEN bit in
the OSC_CR2 register should be set low during PLL opera-
tion. For information on the PLL, refer to the
.
Bit 5: No Buzz.
Normally, when the Sleep bit is set in the
CPU_SCR register, all PSoC device systems are powered
down, including the bandgap reference. However, to facili-
tate the detection of
POR
and
LVD
events at a rate higher
than the sleep interval, the bandgap circuit is powered up
periodically for about 60
s at the Sleep System Duty Cycle
(set in ECO_TR), which is independent of the sleep interval
and typically higher. When the No Buzz bit is set, the Sleep
System Duty Cycle value is overridden and the bandgap cir-
cuit is forced to be on during sleep. This results in a faster
response to an LVD or POR event (continuous detection as
opposed to periodic detection), at the expense of slightly
higher average sleep current.
Bits 4 and 3: Sleep[1:0].
The available sleep interval
selections are shown in
. The accuracy of the
sleep intervals are dependent on the accuracy of the oscilla-
tor used.
Bits 2 to 0: CPU Speed[2:0].
The PSoC M8C may operate
over a range of CPU clock speeds (see
), allow-
ing the M8C’s performance and power requirements to be
tailored to the application.
The reset value for the CPU Speed bits is zero; therefore,
the default CPU speed is one-eighth of the clock source.
The Internal Main Oscillator (IMO) is the default clock
source for the CPU speed circuit; therefore, the default CPU
speed is 3 MHz.
The CPU frequency is changed with a write to the
OSC_CR0 register. There are eight frequencies generated
from a power-of-2 divide circuit, which are selected by a 3-
bit code. At any given time, the CPU 8-to-1 clock mux is
selecting one of the available frequencies, which is resyn-
chronized to the 24 MHz master clock at the output.
Regardless of the CPU Speed bit’s setting, if the actual CPU
speed is greater than 12 MHz, the 24 MHz operating
requirements apply. An example of this scenario is a device
that is configured to use an external clock, which is supply-
ing a frequency of 20 MHz. If the CPU speed register’s value
is 011b, the CPU clock will be 20 MHz. Therefore, the supply
voltage requirements for the device are the same as if the
part was operating at 24 MHz off of the IMO. The operating
voltage requirements are not relaxed until the CPU speed is
at 12 MHz or less.
For additional information, refer to the
.
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Access
1,E0h
32k Select
PLL Mode
No Buzz
Sleep[1:0]
CPU Speed[2:0]
RW : 00
Table 12-1. Sleep Interval Selections
OSC_CR2[4]
Sleep Interval
OSC_CR0[4:3]
Sleep
Timer
Clocks
Sleep
Period
(nominal)
Watchdog
Period
(nominal)
0
00b (default)
64
1.95 ms
6 ms
0
01b
512
15.6 ms
47 ms
0
10b
4,096
125 ms
375 ms
0
11b
32,768
1 sec
3 sec
1
00b (default)
65,536
2 sec
6 sec
1
01b
131,072
4 sec
12 sec
1
10b
262,144
8 sec
24 sec
1
11b
524,288
16 sec
48 sec
Table 12-2. OSC_CR0[2:0] Bits: CPU Speed
Bits
Internal Main Oscillator
External Clock
000b
3 MHz
EXTCLK/ 8
001b
6 MHz
EXTCLK/ 4
010b
12 MHz
EXTCLK/ 2
011b
24 MHz
EXTCLK/ 1
100b
1.5 MHz
EXTCLK/ 16
101b
750 kHz
EXTCLK/ 32
110b
187.5 kHz
EXTCLK/ 128
111b
93.7 kHz
EXTCLK/ 256
Summary of Contents for CY8C28 series
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