PXN20RM Datasheet, PDF (1068/1376 Pages) Freescale Semiconductor, Inc – PXN20 Microcontroller

Inter-Integrated Circuit Bus Controller Module (I²C)
system back into a ready-for-I²C-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 I²C connected via half
speed IPI bus:
• Accessing the I²C from the DMA controller via IPI bus typically requires four cycles (consecutive
accesses to the I²C 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.
32-24
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor