at90pwm81-16se ATMEL Corporation, at90pwm81-16se Datasheet - Page 38

• Manufacturer Part Number: at90pwm81-16se
• Description: 8-bit Avr Microcontroller With 8k Bytes In- System Programmable Flash
• Manufacturer: ATMEL Corporation

6.4
6.4.1
6.4.2
6.5
6.5.1
6.5.2
38
System Clock Prescaler
Register Description
AT90PWM81
Features
Switching Time
OSCCAL – Oscillator Calibration Register
CLKPR – Clock Prescaler Register
The AT90PWM81 system clock can be divided by setting the Clock Prescaler Register – CLKPR. This
feature can be used to decrease power consumption when the requirement for processing power is low.
This can be used with all clock source options, and it will affect the clock frequency of the CPU and all
synchronous peripherals. clk
10 on page
When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occur in
the clock system and that no intermediate frequency is higher than neither the clock frequency corre-
sponding to the previous setting, nor the clock frequency corresponding to the new setting.
The ripple counter that implements the prescaler runs at the frequency of the undivided clock, which may
be faster than the CPU’s clock frequency. Hence, it is not possible to determine the state of the prescaler –
even if it were readable, and the exact time it takes to switch from one clock division to another cannot be
exactly predicted.
From the time the CLKPS values are written, it takes between T1 + T2 and T1 + 2*T2 before the new
clock frequency is active. In this interval, 2 active clock edges are produced. Here, T1 is the previous
clock period, and T2 is the period corresponding to the new prescaler setting.
• Bits 7:0 – CAL7:0: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process
variations from the oscillator frequency. The factory-calibrated value is automatically written to this regis-
ter during chip reset, giving an oscillator frequency of 8.0 MHz at 25°C. The application software can
write this register to change the oscillator frequency. The oscillator can be calibrated to any frequency in
the range 7.6 - 8.4 MHz within ± 1% accuracy. Calibration outside that range is not guaranteed.
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be
affected accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8 MHz. Other-
wise, the EEPROM or Flash write may fail.
The CAL7..0 bits are used to tune the frequency within the selected range. A setting of 0x00 gives the
lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the range. Increment-
ing CAL7..0 by 1 will give a frequency increment of less than 0.5% in the frequency range 7.6 - 8.4 MHz.
Bit
Read/Write
Initial Value
Bit
Read/Write
39.
CLKPCE
CAL7
R/W
R/W
7
7
CAL6
R/W
I/O
6
R
6
–
, clk
ADC
CAL5
R/W
, clk
R
5
5
–
Device Specific Calibration Value
CPU
, and clk
CAL4
R/W
4
R
4
–
FLASH
CLKPS3
CAL3
R/W
R/W
3
3
are divided by a factor as shown in
CLKPS2
CAL2
R/W
R/W
2
2
CLKPS1
CAL1
R/W
R/W
1
1
CLKPS0
CAL0
R/W
R/W
0
0
7734M–AVR–03/10
OSCCAL
CLKPR
Table 6-