ST10F272M-4Q3 STMICROELECTRONICS [STMicroelectronics], ST10F272M-4Q3 Datasheet - Page 150

• Manufacturer Part Number: ST10F272M-4Q3
• Description: 16-bit MCU with 256 Kbyte Flash memory and 20 Kbyte RAM
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Electrical characteristics
24.8.13
Note:
24.8.14
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External clock drive XTAL1
When Direct Drive configuration is selected during reset, it is possible to drive the CPU clock
directly from the XTAL1 pin, without particular restrictions on the maximum frequency, since
the on-chip oscillator amplifier is bypassed. The speed limit is imposed by internal logic that
targets a maximum CPU frequency of 40 MHz.
In all other clock configurations (Direct Drive with Prescaler or PLL usage) the on-chip
oscillator amplifier is not bypassed, so it determines the input clock speed limit. Then, when
the on-chip oscillator is enabled it is forbidden to use any external clock source different
from crystal or ceramic resonator.
Table 68.
1. The minimum value for the XTAL1 signal period is considered the theoretical minimum. The real minimum
2. 4 to 8 MHz is the input frequency range when using an external clock source. 40 MHz can be applied with
3. The input clock signal must reach the defined levels V
Figure 49. External clock drive XTAL1
When Direct Drive is selected, an external clock source can be used to drive XTAL1. The
maximum frequency of the external clock source depends on the duty cycle: When 40 MHz
is used, 50% duty cycle is granted (low phase = high phase = 12.5ns); when for instance
20 MHz is used, a 25% duty cycle can be accepted (minimum phase, high or low, again
equal to 12.5ns).
Memory cycle variables
The tables below use three variables which are derived from the BUSCONx registers and
represent the special characteristics of the programmed memory cycle. The following table
describes how these variables are to be computed.
XTAL1 period
High time
Low time
Rise time
Fall time
value depends on the duty cycle of the input clock signal.
an external clock source only when Direct Drive mode is selected: in this case, the oscillator amplifier is
bypassed so it does not limit the input frequency.
Parameter
(3)
(3)
(3)
(3)
External clock drive
(1)(2)
t
t
t
t
t
OSC
1
2
3
4
Symbol
SR
SR
SR
SR
SR
t
1
Min
25
Direct drive
f
6
6
–
–
CPU
V
= f
IH2
XTAL
Max
–
–
–
2
2
t
2
t
3
IL2
and V
Direct drive with
f
83.3
Min
CPU
t
3
3
–
–
V
OSC
prescaler
IL2
IH2
= f
.
XTAL
Max
250
–
–
2
2
/ 2
f
83.3
CPU
Min
6
6
–
–
PLL usage
t
= f
4
XTAL
ST10F272M
Max
250
–
–
2
2
x F
Unit
ns
ns
ns
ns
ns