AT90USB1286-16MU ATMEL [ATMEL Corporation], AT90USB1286-16MU Datasheet - Page 39

• Manufacturer Part Number: AT90USB1286-16MU
• Description: Microcontroller with 64/128K Bytes of ISP Flash and USB Controller
• Manufacturer: ATMEL [ATMEL Corporation]

6.1.4
6.1.5
6.1.6
6.2
6.2.1
6.2.2
7593A–AVR–02/06
Clock Sources
Asynchronous Timer Clock – clk
ADC Clock – clk
USB Clock – clk
Default Clock Source
Clock Startup Sequence
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly
from an external clock or an external 32 kHz clock crystal. The dedicated clock domain allows
using this Timer/Counter as a real-time counter even when the device is in sleep mode.
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks
in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion
results.
The USB is provided with a dedicated clock domain. This clock is generated with an on-chip PLL
running at 48MHz. The PLL always multiply its input frequency by 24. Thus the PLL clock regis-
ter should be programmed by software to generate a 2MHz clock on the PLL input.
The device has the following clock source options, selectable by Flash Fuse bits as shown
below. The clock from the selected source is input to the AVR clock generator, and routed to the
appropriate modules.
Table 6-1.
Note:
The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 pro-
grammed, resulting in 1.0MHz system clock. The startup time is set to maximum and time-out
period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that
all users can make their desired clock source setting using any available programming interface.
Any clock source needs a sufficient V
cycles before it can be considered stable.
To ensure sufficient V
the device reset is released by all other reset sources.
describes the start conditions for the internal reset. The delay (t
Oscillator and the number of cycles in the delay is set by the SUTx and CKSELx fuse bits. The
selectable delays are shown in
ADC
USB
Device Clocking Option
Low Power Crystal Oscillator
Reserved
Low Frequency Crystal Oscillator
Internal 128 kHz RC Oscillator
Calibrated Internal RC Oscillator
External Clock
Reserved
1. For all fuses “1” means unprogrammed while “0” means programmed.
Device Clocking Options Select
ASY
CC
, the device issues an internal reset with a time-out delay (t
Table
CC
6-2. The frequency of the Watchdog Oscillator is voltage
to start oscillating and a minimum number of oscillating
(1)
“On-chip Debug System” on page 57
TOUT
AT90USB64/128
) is timed from the Watchdog
0101 - 0100
1111 - 1000
0111 - 0110
CKSEL3..0
0010
0000
0001
0011
TOUT
) after
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