Option bytes (78K0/Kx2, 78K0S/Kx1+)
All series of microcontrollers (MCUs) share the same basic functions (instruction set, architecture, etc.), but some of their on-chip functions differ according to the package (pin number), and even different models that use the same package can have different amounts of on-chip memory (flash memory, RAM, etc.). In conventional microcontrollers, detailed functions (such as port pull-up resistance and POC voltage) have been specified as mask options when writing programs to ROM. However, since PROM devices and flash memory devices do not have mask program writing processes, they cannot have mask options. As for most port pull-up resistances, these can be set by software, but there are still functions such as POC voltage that cannot be set by software.
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When a port's pull-up resistance can be set by software, it greatly increases freedom in using pull-up resistance. In particular, products that feature on-chip flash memory can be modified after they are shipped. On the other hand, they are not valid as soon as the power is turned ON, as are mask options. Resistance does not become valid until the software that sets the pull-up resistance is executed. Since the POC voltage specifies the supply voltage used to enable execution of software, it is in principle impossible to set it by software. It is therefore important to make the distinction between what can and cannot be set by software.
[Support by flash devices] Products such as the 78K0/Kx1 microcontrollers that include mask devices and flash memory devices, offer a separate lineup of products that support mask options. See the description of the 78K0/Kx1 under the flash devices section of this FAQ. In such cases, when the product needs only to have certain mask options it is important to select precisely to obtain the desired functions. In particular, the devices that serve the desired objective may change due to revisions in related specifications, in which case different devices must be selected. A new support approach called "option byte" is used in all-flash 8-bit microcontrollers such as the 78K0/Kx2 and the 78K0S/Kx1+. The available options and the specification format differ among different devices.
[Option byte] The option byte is assigned to a specified address in the microcontroller's on-chip flash memory (the address that is used is the start address for the ordinary program memory, which starts at 80H). When the microcontroller is started, such as by power-on or reset, first the option byte setting is referenced, and the setting is reflected in the microcontroller's functions. Next, the application program starts from the address indicated by the reset vector at address 0.