Programming Model

The RDAR register is cleared at reset and when ECR[ETHER_EN] is cleared.

Field

Reset

R/W

Field

Reset

R/W Address

31

25

24

23

16

 

R_DES_ACTIVE

 

 

 

 

 

 

 

0000_0000_0000_0000

 

 

 

 

 

 

 

 

 

R/W

 

 

 

 

 

 

 

15

 

 

 

0

0000_0000_0000_0000

R/W

IPSBAR + 0x1010

Figure 17-6. Receive Descriptor Active Register (RDAR)

Table 17-14. RDAR Field Descriptions

Bits

Name

Description

 

 

 

31–25

Reserved, should be cleared.

 

 

 

24

R_DES_ACTIVE

Set to one when this register is written, regardless of the value written.

 

 

Cleared by the FEC device whenever no additional “empty” descriptors

 

 

remain in the receive ring. Also cleared when ECR[ETHER_EN] is cleared.

 

 

 

23–0

Reserved, should be cleared.

 

 

 

17.5.4.4 Transmit Descriptor Active Register (TDAR)

The TDAR is a command register which should be written by the user to indicate that the transmit descriptor ring has been updated (transmit buffers have been produced by the driver with the ready bit set in the buffer descriptor).

Whenever the register is written, the TDAR bit is set. This value is independent of the data actually written by the user. When set, the FEC will poll the transmit descriptor ring and process transmit frames (provided ECR[ETHER_EN] is also set). Once the FEC polls a transmit descriptor whose ready bit is not set, then the FEC will clear the TDAR bit and cease transmit descriptor ring polling until the user sets the bit again, signifying additional descriptors have been placed into the transmit descriptor ring.

The TDAR register is cleared at reset, when ECR[ETHER_EN] is cleared, or when ECR[RESET] is set.

MOTOROLA

Chapter 17. Fast Ethernet Controller (FEC)

17-27

Page 383
Image 383
Motorola MCF5281, MCF5282 user manual Transmit Descriptor Active Register Tdar, Rdesactive