NXP Semiconductors LPC24XX UM10237 - Manual (Page 250)

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User manual Rev. 04 — 26 August 2009 250 of 792
NXP Semiconductors
UM10237
Chapter 11: LPC24XX Ethernet
Ethernet block has finished reading/writing the last descriptor/status of the array (with the
highest memory address), the next descriptor/status it reads/writes is the first
descriptor/status of the array at the base address of the array.
Full and Empty state of descriptor arrays
The descriptor arrays can be empty, partially full or full. A descriptor array is empty when
all descriptors are owned by the producer. A descriptor array is partially full if both
producer and consumer own part of the descriptors and both are busy processing those
descriptors. A descriptor array is full when all descriptors (except one) are owned by the
consumer, so that the producer has no more room to process frames. Ownership of
descriptors is indicated with the use of a consume index and a produce index. The
produce index is the first element of the array owned by the producer. It is also the index
of the array element that is next going to be used by the producer of frames (it may
already be busy using it and subsequent elements). The consume index is the first
element of the array that is owned by the consumer. It is also the number of the array
element next to be consumed by the consumer of frames (it and subsequent elements
may already be in the process of being consumed). If the consume index and the produce
index are equal, the descriptor array is empty and all array elements are owned by the
producer. If the consume index equals the produce index plus one, then the array is full
and all array elements (except the one at the produce index) are owned by the consumer.
With a full descriptor array, still one array element is kept empty, to be able to easily
distinguish the full or empty state by looking at the value of the produce index and
consume index. An array must have at least 2 elements to be able to indicate a full
descriptor array with a produce index of value 0 and a consume index of value 1. The
wrap around of the arrays is taken into account when determining if a descriptor array is
full, so a produce index that indicates the last element in the array and a consume index
that indicates the first element in the array, also means the descriptor array is full. When
the produce index and the consume index are unequal and the consume index is not the
produce index plus one (with wrap around taken into account), then the descriptor array is
partially full and both the consumer and producer own enough descriptors to be able to
operate actively on the descriptor array.
Interrupt bit
The descriptors have an Interrupt bit, which is programmed by software. When the
Ethernet block is processing a descriptor and finds this bit set, it will allow triggering an
interrupt (after committing status to memory) by passing the RxDoneInt or TxDoneInt bits
in the IntStatus register to the interrupt output pin. If the Interrupt bit is not set in the
descriptor, then the RxDoneInt or TxDoneInt are not set and no interrupt is triggered (note
that the corresponding bits in IntEnable must also be set to trigger interrupts). This offers
flexible ways of managing the descriptor arrays. For instance, the device driver could add
10 frames to the Tx descriptor array, and set the Interrupt bit in descriptor number 5 in the
descriptor array. This would invoke the interrupt service routine before the transmit
descriptor array is completely exhausted. The device driver could add another batch of
frames to the descriptor array, without interrupting continuous transmission of frames.
Frame fragments
For maximum flexibility in frame storage, frames can be split up into multiple frame
fragments with fragments located in different places in memory. In this case one
descriptor is used for each frame fragment. So, a descriptor can point to a single frame or