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11-74 SA-1100
Developer’s Manual
Peripheral Control Module
11.8.10 UDC Endpoint 0 Data Register
The UDC endpoint 0 data register is actually an 8-bit x 8-entry bidirectional FIFO. When the host
transmits data to the UDC endpoint 0, the CPU reads the UDC endpoint 0 register to access the
data. When the UDC is sending data to the host, the CPU writes the data to be sent into the UDC
endpoint 0 register. Although the same FIFO can be read and written by the CPU during various
points in a control sequence, the CPU may not read and write the FIFO at the same time. The
direction that the FIFO is flowing is controlled by the UDC. Normally, the UDC will be in an idle
state, waiting for the host to send commands. When this happens, the UDC fills the FIFO with the
command from the host and the CPU reads the command from the FIFO once it has arrived. The
UDC will do a partial decode of the command to determine if the CPU is going to be filling the
FIFO with data to send to the host. If so, the direction is turned around to accept data from the CPU
and have the UDC transmit the data. If the command is such that no data will be required from the
UDC, then this will not happen. The only time the CPU may write the endpoint 0 FIFO is when a
valid command from the host has been received which requires a transmission in response, that is,
a GET_DESCRIPTOR command.
11.8.11 UDC Endpoint 0 Write Count Register
The UDC endpoint 0 write count register can be read when a packet has been received by the
endpoint 0 to determine how many bytes to read out of the UDC endpoint 0 data register. When
data is present in the FIFO, this 4-bit field should read between 1 and 8.
Address: 0h 8000 001C UDCD0 Read/Write
Bit76543210
Bottom of Endpoint 0 FIFO
Reset00000000
Read Access
Bit76543210
Top of Endpoint 0 FIFO
Reset00000000
Write Access
Bit Name Description
7..0 DATA
Top/bottom of endpoint 0 FIFO data.
Read – Bottom of endpoint 0 FIFO data.
Write – Top of endpoint 0 FIFO data.
Address: 0h 8000 0020 UDCWC Read-Only
Bit76543210
Reserved Write Count
Reset00000000
Bit Name Description
3..0 WC
Endpoint 0 write count (read-only).
4-bit field representing the number of bytes in the endpoint 0 FIFO.
7..4 —
Reserved.
Always reads zero.