Datasheet

496
32117D–AVR-01/12
AT32UC3C
64 1s, whenever it sees an incoming frame to force a collision. This provides a way of imple-
menting flow control in half-duplex mode.
24.5.6 Pause Frame Support
The start of an 802.3 pause frame is as follows:
The network configuration register contains a receive pause enable bit (13). If a valid pause
frame is received, the pause time register is updated with the frame’s pause time, regardless of
its current contents and regardless of the state of the configuration register bit 13. An interrupt
(12) is triggered when a pause frame is received, assuming it is enabled in the interrupt mask
register. If bit 13 is set in the network configuration register and the value of the pause time reg-
ister is non-zero, no new frame is transmitted until the pause time register has decremented to
zero.
The loading of a new pause time, and hence the pausing of transmission, only occurs when the
MACB is configured for full-duplex operation. If the MACB is configured for half-duplex, there is
no transmission pause, but the pause frame received interrupt is still triggered.
A valid pause frame is defined as having a destination address that matches either the address
stored in specific address register 1 or matches 0x0180C2000001 and has the MAC control
frame type ID of 0x8808 and the pause opcode of 0x0001. Pause frames that have FCS or other
errors are treated as invalid and are discarded. Valid pause frames received increment the
Pause Frame Received statistic register.
The pause time register decrements every 512 bit times (i.e., 128 RX_CLK in nibble mode) once
transmission has stopped. For test purposes, the register decrements every RX_CLK cycle once
transmission has stopped if bit 12 (retry test) is set in the network configuration register. If the
pause enable bit (13) is not set in the network configuration register, then the decrementing
occurs regardless of whether transmission has stopped or not.
An interrupt (13) is asserted whenever the pause time register decrements to zero (assuming it
is enabled in the interrupt mask register). Automatic transmission of pause frames is supported
through the transmit pause frame bits of the network control register and the tx_pause and
tx_pause_zero inputs. If either bit 11 or bit 12 of the network control register is written to with a 1,
or if the input signal tx_pause is toggled, a pause frame is transmitted only if full duplex is
selected in the network configuration register and transmit is enabled in the network control
register.
Pause frame transmission occurs immediately if transmit is inactive or if transmit is active
between the current frame and the next frame due to be transmitted. The transmitted pause
frame is comprised of the items in the following list:
• a destination address of 01-80-C2-00-00-01
• a source address taken from the specific address 1 register
• a type ID of 88-08 (MAC control frame)
• a pause opcode of 00-01
• a pause quantum
Table 24-3. Start of an 802.3 Pause Frame
Destination Address
Source
Address
Type
(Mac Control Frame)
Pause
Opcode Pause Time
0x0180C2000001 6 bytes 0x8808 0x0001 2 bytes