C8051F320/1
146 Rev. 1.4
15.4. 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 fu
nction, the USB0 clock must be 48 MHz. Clock options
are described in Section “13. Oscillators” on page 116. The USB0 clock is selected
via SFR CLKSEL (see
Figure 13.5 on Page 124).
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:
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 Recove
ry circuitry to lock when high noise levels are pres-
ent on the USB network. This mode is not require
d (or recommended) in typical USB environments.
USB Register Definition 15.5. CLKREC: Clock Recovery Control
Communication Speed USB Clock 4x Clock Multiplier Input
Full Speed 4x Clock Multiplier Internal Oscillator
Low Speed Internal Oscillator/2 N/A
Bit7: 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 = 1001b.
R/W R/W R/W R/W R/W R/W R/W R/W Reset Value
CRE CRSSEN CRLOW Reserved 00001001
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0 USB Address:
0x0F
