NXP Semiconductors PXN2020, Reference Manual, Page: 1068 / 1376

Inter-Integrated Circuit Bus Controller Module (I
2
C)
PXN20 Microcontroller Reference Manual, Rev. 1
32-24 Freescale Semiconductor
system back into a ready-for-I
2
C-transfer state. The advantage over the two other solutions is that
this does not require CPU intervention or a second DMA channel. This comes at the cost of 64
bytes RAM (two TCDs), some system bus transfer overhead, and a little increase in application
code complexity. On the system level, no higher priority DMA requests must occur during the
scatter-gather process because those can result in a slow reaction.
Example latencies for a 32 MHz system with a full speed 32-bit AHB bus and an I
2
C connected via half
speed IPI bus:
• Accessing the I
2
C from the DMA controller via IPI bus typically requires four cycles (consecutive
accesses to the I
2
C could be faster):
4 x T
IPI
= 4 / 16 MHz = 250 ns Eqn. 32-6
• Reloading a new TCD (8

32 bit) via AHB to the DMA controller (scatter/gather process):
8 x T
AHB
= 8 / 32 MHz = 250 ns Eqn. 32-7
With the DMA scatter-gather process, the required IBCR access can be done in 0.5

s, leaving a large
margin of 19.5

s for additional system delays. The slow reaction case can be prevented in this way. The
system user must decide which usage model suits his overall requirements best.