Do a 8-bit write to a 32-bit register

Question

I'm trying to read the clock source value of a given generic clock generator on the Samd21 MCU.

The datasheet says, that if I wish to read the GENCTRL register (containing the clock source value), I need to "do a 8-bit write" and read the register afterwards. How can I do that, given that the register is 32-bit?

I'm afraid, that by doing the following, I am actually changing the generic clock generator X 's configuration:

GCLK->GENCTRL.reg = GCLK->GENCTRL.reg & 0xFFFFFFF0 | 0x0000000X

Keep in mind, that the lower 8-bits of GENCTRL are reserved for the generic clock generator's ID.

Bellow is part of the datasheet containing the instructions for reading the GENCTRL register.

Answer 1

The ARM registers are 32 bit. The peripheral registers (in general) will be arranged at 4 byte offsets, but will not always implement all 32 bits that this implies.

This is most obvious when the upper bits of a peripheral register are 'read as zero, write ignored'. You might occasionally see a newer or more featured version of the peripheral where some of these unused bits become used in the future.

Depending on exactly how a specific peripheral is connected to the core it is generally possible to perform byte, half-word or word accesses to any region of memory. Provided this is supported, only the relevant bytes will be updated. Where there is a restriction (for example a 32 bit APB bus where only byte access is supported), this should be clearly identified in the documentation. With a AA64 processor, it is even possible to write two registers at once!

Do note that the peripheral 'knows' the access size (at least the information is present on the internal bus), so it is possible to specify different behaviour for a byte access as a word (even if this is the sort of confusing behaviour that is best avoided). To generalise, any memory mapped peripheral is more of an observer of the bus than a true implementation of memory - the designer is free to play tricks with the full address/data/control bus bit combinations, and implement bitmasks, read/modify/write, access locks, magic values, etc.