Controlling The 82557 - Intel Embedded Intel486 Hardware Reference Manual

PERIPHERAL SUBSYSTEM
7.6.2.6

Controlling the 82557

The CPU issues control commands to the Command Unit (CU) and Receive Unit (RU) through
the SCB, which is part of the CSR. The CPU instructs the 82557 to Activate, Suspend, Resume
or Idle the CU or RU by placing the appropriate control command in the CU or RU control field.
A CPU write access to the SCB causes the 82557 to read the SCB, including the Status word,
Command word, CU and RU Control fields, and the SCB General Pointer. Activating the CU
causes the 82557 to begin executing the CBL. When execution is complete, the 82557 updates
the SCB with the CU status, then interrupts the CPU, if configured to do so. Activating the RU
causes the 82557 to access the RFA and go into the READY state for frame reception. When a
frame is received, the RU updates the SCB with the RU status and interrupts the CPU. It also au-
tomatically advances to the next free RFD in the RFA. This interaction between the CPU and
82557 can continue until a software reset is issued to the 82557, at which point the initialization
process must be executed again. The CPU can also perform certain 82557 functions directly
through a CPU PORT interface.
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