I am reading the STM32Lxxxx Reference manual (RM0394 Rev 4, October 2018, DM00151940).
On page 267, section 8.4 "GPIO registers" says "The peripheral registers can be written in word, half word or byte mode." Sure enough, any access works.
On page 308, section 11.6 "DMA registers" says "The DMA registers have to be accessed by words (32-bit)." Sure enough, if I attempt to access them by byte or half-word, I get an exception - I suspect Bus Error, but I didn't look too deeply.
However, the document says nothing about accesses to many other peripherals. For example, if I do a byte write to USART_CR1 to enable the USART (UE=0b1), I don't get an exception. Instead, the register also sets the bottom bit of the other three bytes (PEIE, DEDT[0] and DEAT[3]) in the register!
This is similar to what is described in section 11.4.5 "Addressing AHB peripherals not supporting byte/half-word write transfers":
"...any AHB byte or half-word transfer is changed into a 32-bit APB
transfer as described below:
• An AHB byte write transfer of 0xB0 to one of the 0x0, 0x1, 0x2 or 0x3 addresses, is
converted to an APB word write transfer of 0xB0B0B0B0 to the 0x0 address.
• An AHB half-word write transfer of 0xB1B0 to the 0x0 or 0x2 addresses, is converted to
an APB word write transfer of 0xB1B0B1B0 to the 0x0 address."
My question is: Should I ensure that all register accesses are full words all of the time (except where the document explicitly says I can use bytes)? A perfect example is USART_RDR and USART_TDR (page 1259). Only the bottom 9 bits are defined, and I want to be able to read and write byte characters. But that means that I'd be setting some of the high Reserved bits. Do I need to zero-extend the character to a full word before writing the whole register?
