Rainbow Electronics AT89C51SND1C - Manual (Page 81)

81
AT8xC51SND1C
4109E–8051–06/03
Description The USB device controller provides the hardware that the AT8xC51SND1C needs to
interface a USB link to a data flow stored in a double port memory.
It requires a 48 MHz reference clock provided by the clock controller as detailed in Sec-
tion "Clock Controller", page 81. This clock is used to generate a 12 MHz Full Speed bit
clock from the received USB differential data flow and to transmit data according to full
speed USB device tolerance. Clock recovery is done by a Digital Phase Locked Loop
(DPLL) block.
The Serial Interface Engine (SIE) block performs NRZI encoding and decoding, bit stuff-
ing, CRC generation and checking, and the serial-parallel data conversion.
The Universal Function Interface (UFI) controls the interface between the data flow and
the Dual Port RAM, but also the interface with the C51 core itself.
Figure 55. USB Device Controller Block Diagram
Clock Controller The USB controller clock is generated by division of the PLL clock. The division factor is
given by USBCD1:0 bits in USBCLK register (see Table 104). Figure 56 shows the USB
controller clock generator and its calculation formula. The USB controller clock fre-
quency must always be 48 MHz.
Figure 56. USB Clock Generator and Symbol
USB
CLOCK
48 MHz 12 MHz
D+
D-
DPLL
SIE
UFI
USB
Buffer
To/From
C51 Core
USBCD1:0
USBCLK
48 MHz USB Clock
USBclk
PLLclk
USBCD 1 +
-------------------------------- =
USB
CLOCK
USB Clock Symbol
PLL
CLOCK