EFTON - STM32 gotchas

106. Bit-banding cannot be used with DMA

Bit-banding is an interesting addition to the Cortex-M3/M4 processors (used in STM32F1/L1/F2/F4/F3/L4/G4), which allows atomic access¹ to a portion of memory and peripherals.

In some cases, it is tempting to use this access to read or write single bits also using DMA (e.g. to flip polarity of TIMx_CHx output by writing to its respective TIMx_CCER.CCxP).

This does not work, as bit-banding is performed by an extension on the S-port of the processor. This extension intercepts accesses from processor targeted to the bit-addressable regions, "translates" the address from the bit-addressable to the "real" memory/peripheral address, generates the appropriate bitmask, and converts the bit-banding access to either a RMW operation modifying the required bit if it is a write, or to a read followed by mask-and-shift to return only the required bit on the data bus for the processor.

However, accesses from DMA don't go through this extension.

In other words, for DMA (and other bus-masters), the bit-addressable alias area is simply nonexistent, and accessing it results in DMA transfer error.

1. Bit-oriented access through bit-banding is atomic from the processor's perspective, i.e. it cannot be interrupted by any other process within the processor. However, if the target of access is a bit within a peripheral register with other bits changed directly by hardware, the consequences are the same as with any other read-modify-write accesses to such peripheral registers. See detailed discussion of this issue here.