C8051F340/1/2/3/4/5/6/7
16.4. USB Clock Configuration
USB0 is capable of communication as a Full or Low Speed USB function. Communication speed is
selected via the SPEED bit in SFR USB0XCN. When operating as a Low Speed function, the USB0 clock
must be 6 MHz. When operating as a Full Speed function, the USB0 clock must be 48 MHz. Clock options
are described in
. The USB0 clock is selected via SFR CLKSEL
(see SFR Definition 14.6).
Clock Recovery circuitry uses the incoming USB data stream to adjust the internal oscillator; this allows
the internal oscillator (and 4x Clock Multiplier) to meet the requirements for USB clock tolerance. Clock
Recovery should be used in the following configurations:
Communication Speed
Full Speed
Low Speed
USB Clock
4x Clock Multiplier
Internal Oscillator / 2
4x Clock Multiplier Input
Internal Oscillator
N/A
When operating USB0 as a Low Speed function with Clock Recovery, software must write ‘1’ to the
CRLOW bit to enable Low Speed Clock Recovery. Clock Recovery is typically not necessary in Low Speed
mode.
Single Step Mode can be used to help the Clock Recovery circuitry to lock when high noise levels are
present on the USB network. This mode is not required (or recommended) in typical USB environments.
USB Register Definition 16.5. CLKREC: Clock Recovery Control
R/W
R/W
R/W
R/W
Bit4
R/W
Bit3
R/W
R/W
Bit1
R/W
Bit0
Reset Value
CRE
Bit7
CRSSEN
Bit6
CRLOW
Bit5
Reserved
Bit2
00001001
USB Address:
0x0F
CRE: Clock Recovery Enable.
This bit enables/disables the USB clock recovery feature.
0: Clock recovery disabled.
1: Clock recovery enabled.
Bit6:
CRSSEN: Clock Recovery Single Step.
This bit forces the oscillator calibration into ‘single-step’ mode during clock recovery.
0: Normal calibration mode.
1: Single step mode.
Bit5:
CRLOW: Low Speed Clock Recovery Mode.
This bit must be set to ‘1’ if clock recovery is used when operating as a Low Speed USB
device.
0: Full Speed Mode.
1: Low Speed Mode.
Bits4–0: Reserved. Read = Variable. Must Write = 01001b.
Bit7:
Note:
The USB transceiver must be enabled before enabling Clock Recovery.
174
Rev. 1.0
SILABS [ SILICON LABORATORIES ]
