8.5.6 Accessing 16-bit Registers

Most of the registers for the ATtiny417/814/816/817 devices are 8-bit registers, but the devices also features a few 16-bit registers. As the AVR data bus has a width of 8 bits, accessing the 16-bit requires two read or write operations. All the 16-bit registers of the ATtiny417/814/816/817 devices are connected to the 8-bit bus through a temporary (TEMP) register.

For a 16-bit write operation, the low byte register (e.g. DATAL) of the 16-bit register must be written before the high byte register (e.g. DATAH). Writing the low byte register will result in a write to the temporary (TEMP) register instead of the low byte register, as shown in the left side of Figure 8-6. When the high byte register of the 16-bit register is written, TEMP will be copied into the low byte of the 16-bit register in the same clock cycle, as shown in the right side of Figure 8-6.

For a 16-bit read operation, the low byte register (e.g. DATAL) of the 16-bit register must be read before the high byte register (e.g. DATAH). When the low byte register is read, the high byte register of the 16-bit register is copied into the temporary (TEMP) register in the same clock cycle, as show in the left side of Figure 8-7. Reading the high byte register will result in a read from TEMP instead of the high byte register, as shown in right side of Figure 8-7.

The described mechanism ensures that the low and high bytes of 16-bit registers are always accessed simultaneously when reading or writing the registers.

Interrupts can corrupt the timed sequence if an interrupt is triggered during a 16-bit read/write operation and a 16-bit register within the same peripheral is accessed in the interrupt service routine. To prevent this, interrupts should be disabled when writing or reading 16-bit registers. Alternatively, the temporary register can be read before and restored after the 16-bit access in the interrupt service routine.