3.2 Register Initialization

To initialize a register, the user must find the desired configuration by consulting the device data sheet. Then, the register bits must be set or cleared so that the value in the register matches the desired configuration.

Register initialization is most often performed as part of device initialization after Reset when the register is in a known Reset state (default).

The Reset value for all bits and bit fields is ‘0’ for most AVR registers. Figure 3-1 shows how to find the Reset value and all the register’s bits configurations desired for this example.

Figure 3-1. ADC Control A Register - Reset Value and Bit Settings

Read-Modify-Write operations are not needed when working with bit masks or -positions if the Reset value of the register is 0x00, and the register can be configured in a single line.

The desired configuration can be, for example:

  • ADC is enabled – the ENABLE bit will be ‘1’
  • ADC is operating in Differential mode – the CONVMODE bit will be ‘1’
  • ADC will run with 10-bit resolution – the RESSEL bit field value will be 0x00 (default)

This configuration can be implemented by using either the binary, hexadecimal, or decimal value (as presented below) and writing the resulting value directly to the register.

ADC0.CTRLA = 0b00100001;   /* binary */
ADC0.CTRLA = 0x21;         /* hexadecimal */
ADC0.CTRLA = 33;           /* decimal */

However, to improve the code readability (and potentially the portability), it is recommended to use the device defines shown in the upcoming sections.

Note: For AVR registers, the Reset value for most bits and bit fields is ‘0’, but there are exceptions. For example, the USART0 Control C register has a couple of bits with the Reset value ‘1’. In this case, the user must explicitly set the desired configuration without relying on the fact that the Reset values of the bits are usually ‘0’.
Figure 3-2. USART Control C Register – Reset Value

The USART0 Control C register has a Reset value of 0x03. In this case, there is a need for a Read-Modify-Write operation to initialize one of the other bits or bit fields without changing the value of the CHSIZE bit field. Figure 3-2 shows this register.