Local Host (Irq/Polling) Mode - Texas Instruments OMAP5910 Technical Reference Manual

MMC/SD Host Controller

7.12.10 Local Host (IRQ/Polling) Mode

7.12.10.1 MMC Local Host (IRQ/Polling) Receive Mode
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During a local host block read operation (single or multiple) using inerrupt/poll-
ing mode, the A_Full status bit is set active (high level) when the FIFO level
becomes equal or greater than the threshold set in AF_level. The local host
can only clear the A_Full set bit by writing a '1' into the A_Full status bit location.
The threshold level (AF_level) equals LH (local host) transfer size (n) minus
1. If the threshold is set to 0 (AF_level=0x00), the local host has to read one
word by one word. If the threshold is set to 31 (AF_level=0x1F), the local host
has to read 32 words by 32 words.
New A_Full status bit assertion (high level) is internally masked until the LH
has not performed exactly n reads.
Unlike DMA mode, it is not needed to have the total data size of the transfer
be a multiple of the LH read access size (maximum 32 words). The last LH read
access can be down to the LH transfer size when ending any total data size
transfer, as follows:
LH transfer size = n =FIFO size (max 32 16–bit words)
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AF_level = n –1 (FIFO threshold level)
An example is a multiple block read of 10 blocks of 512 bytes each.
The LH transfer size n can be set to 20 words (40 bytes) and AF_level= 0x13
(0x14–1). Then the LH read transfer operation completes after 128 A_Full sta-
tus bit assertions.
The LH transfer size n can also be set to 15 words (30 bytes) and AF_level =
0x0E (0x0F–1). Then the LH read transfer operation completes after 170
system A_Full status bit assertion/deassertions, plus a last assertion for the
last transfer of 10 words (20bytes).
The receive FIFO never overflows. If the FIFO gets full, the MMC_clk clock
signal is momentarily stopped untll the system DMA or the LH performs a read
access, which starts emptying the FIFO.