GPMC
7.1.3.3.12.4.3 FIFO Control in Prefetch Mode
The FIFO can be drained directly by the MPU or by an eDMA channel.
In MPU draining mode, the FIFO status can be monitored through the GPMC_PREFETCH_STATUS[30-
24] FIFOPOINTER field or through the GPMC_PREFETCH_STATUS[16] FIFOTHRESHOLDSTATUS bit.
The FIFOPOINTER indicates the current number of available data to be read; FIFOTHRESHOLDSTATUS
set to 1 indicates that at least FIFOTHRESHOLD bytes are available from the FIFO.
An interrupt can be triggered by the GPMC if the GPMC_IRQENABLE[0] FIFOEVENTENABLE bit is set.
The FIFO interrupt event is logged, and the GPMC_IRQSTATUS[0] FIFOEVENTSTATUS bit is set. To
clear the interrupt, the MPU must read all the available bytes, or at least enough bytes to get below the
programmed FIFO threshold, and the FIFOEVENTSTATUS bit must be cleared to enable further interrupt
events. The FIFOEVENTSTATUS bit must always be reset prior to asserting the FIFOEVENTENABLE bit
to clear any out-of-date logged interrupt event. This interrupt generation must be enabled after enabling
the STARTENGINE bit.
Prefetch completion can be monitored through the GPMC_PREFETCH_STATUS[13-0] COUNTVALUE
field. COUNTVALUE indicates the number of currently remaining data to be requested according to the
TRANSFERCOUNT value. An interrupt can be triggered by the GPMC when the prefetch process is
complete (that is, COUNTVALUE equals 0) if the GPMC_IRQENABLE[1]
TERMINALCOUNTEVENTENABLE bit is set. At prefetch completion, the TERMINALCOUNT interrupt
event is also logged, and the GPMC_IRQSTATUS[1] TERMINALCOUNTSTATUS bit is set. To clear the
interrupt, the MPU must clear the TERMINALCOUNTSTATUS bit. The TERMINALCOUNTSTATUS bit
must always be cleared prior to asserting the TERMINALCOUNTEVENTENABLE bit to clear any out-of-
date logged interrupt event.
NOTE:
The COUNTVALUE value is only valid when the prefetch engine is active (started), and an
interrupt is only triggered when COUNTVALUE reaches 0, that is, when the prefetch engine
automatically goes from an active to an inactive state.
The number of bytes to be prefetched (programmed in TRANSFERCOUNT) must be a multiple of the
programmed FIFOTHRESHOLD to trigger the correct number of interrupts allowing a deterministic and
transparent FIFO control. If this guideline is respected, the number of ISR accesses is always required
and the FIFO is always empty after the last interrupt is trigerred. In other cases, the TERMINALCOUNT
interrupt must be used to read the remaining bytes in the FIFO (the number of remaining bytes being
lower than the FIFOTHRESHOLD value).
In DMA draining mode, the GPMC_PREFETCH_CONFIG1[2] DMAMODE bit must be set so that the
GPMC issues a DMA hardware request when at least FIFOTHRESHOLD bytes are ready to be read from
the FIFO. The DMA channel owning this DMA request must be programmed so that the number of bytes
programmed in FIFOTHRESHOLD is read from the FIFO during the DMA request process. The DMA
request is kept active until this number of bytes has effectively been read from the FIFO, and no other
DMA request can be issued until the ongoing active request is complete.
In prefetch mode, the TERMINALCOUNT event is also a source of DMA requests if the number of bytes
to be prefetched is not a multiple of FIFOTHRESHOLD, the remaining bytes in the FIFO can be read by
the DMA channel using the last DMA request. This assumes that the number of remaining bytes to be
read is known and controlled through the DMA channel programming model.
Any potentially active DMA request is cleared when the prefetch engine goes from inactive to active
prefetch (the STARTENGINE bit is set to 1). The associated DMA channel must always be enabled by the
MPU after setting the STARTENGINE bit so that the out-of-date active DMA request does not trigger
spurious DMA transfers.
330
Memory Subsystem
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated
