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24. Digital Clocks
CY8C22xxx Preliminary Data Sheet
256
Document No. 38-12009 Rev. *D
December 22, 2003
24.2
Register Definitions
24.2.1
INT_CLR0 Register
The INT_CLR0 register holds bits that are used by several
different resources. The digital clocks only use bit 7 of the
INT_CLR0 register for the VC3 clock and bits zero through
six are used by other resources. For a full discussion of the
INT_CLR0 register, see the
INT_CLRx Register
in the Inter-
rupt Controller chapter.
For additional information, reference the
INT_CLR0 register
on page 129
.
24.2.2
INT_MSK0 Register
The INT_MSK0 register holds bits that are used by several
different resources. The digital clocks only use bit 7 of the
INT_MSK0 register for the VC3 clock and bits zero through
six are used by other resources. The Sleep bit (bit 6) con-
trols whether the Sleep timer may be used as an interrupt
source. For a full discussion of the INT_MSK0 register, see
the
INT_MSKx Register
in the Interrupt Controller chapter.
For additional information, reference the
INT_MSK0 register
on page 134
.
24.2.3
OSC_CR0 Register
Bit 7: 32k Select.
By default, the 32 kHz clock source is
the Internal Low-Speed Oscillator (ILO). Optionally, the
External Crystal Oscillator (ECO) may be selected.
Bit 6: PLL Mode.
This bit, in the OSC_CR0 register, is the
only bit that directly influences the PLL. When set, this bit
enables the PLL. The EXTCLK bit should be set low during
PLL operation.
Bit 5: No Buzz.
Normally, when the Sleep bit is set in the
CPU_SCR register, all chip systems are powered down,
including the Band Gap reference. However, to facilitate the
detection of POR and LVD events at a rate higher than the
Sleep Interval, the Band Gap circuit is powered up periodi-
cally for about 60 us 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 Sys-
tem Duty Cycle value is overridden, and the Band Gap cir-
cuit is forced to be on during sleep. This results in faster
response to an LVD or POR event (continuous detection as
opposed to periodic), at the expense of slightly higher aver-
age sleep current.
Bits 4 and 3: Sleep[1:0].
The available sleep interval
selections are shown in
Table 24-2
. It must be remembered
that when the ILO is the selected 32 kHz clock source, sleep
intervals are approximate.
Bits 2, 1, and 0: CPU Speed[2:0].
The PSoC M8C may
operate over a range of CPU clock speeds (
Table 24-3
),
allowing 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 is the default clock source for
the CPU speed circuit; therefore, the default CPU speed is 3
MHz. See
“External Clock” on page 254
for more informa-
tion on the supported frequencies for externally supplied
clocks.
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 8:1 clock multiplexer is
selecting one of the available frequencies, which is re-syn-
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 0b011, 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 internal
main oscillator. The operating voltage requirements are not
relaxed until the CPU speed is at 12.0 MHz or less.
For additional information, reference the
OSC_CR0 register
on page 165
.
Table 24-2. Sleep Interval Selections
Sleep Interval
OSC_CR[4:3]
00b (default)
01b
10b
11b
Sleep Timer
Clocks
64
512
4096
32,768
Sleep Period
(nominal)
1.95 ms
15.6 ms
125 ms
1 sec
Watchdog
Period
(nominal)
6 ms
47 ms
375 ms
3 sec
Table 24-3. OSC_CR0[2:0] Bits: CPU Speed
Divider Source Clock
Bits
000b
001b
010b
011b
100b
101b
110b
111b
Internal Main Oscillator
3 MHz
6 MHz
12 MHz
24 MHz
1.5 MHz
750 kHz
187.5 kHz
93.7 kHz
External Clock
EXTCLK/ 8
EXTCLK/ 4
EXTCLK/ 2
EXTCLK/ 1
EXTCLK/ 16
EXTCLK/ 32
EXTCLK/ 128
EXTCLK/ 256