The two button mode-set user interface is a design to allow changing many settings with only two buttons. It is used in many devices, even my ancient Casio F91-W watch from 45 years ago used it. Here is a simple example used in a 4 digit clock:

As you can see, button 1 loops through the various modes, and button 2 is an increment button, which also loops on wrap around. Sometimes a third button is provided to decrement the counter, but I feel this detracts from simplicity. With autorepeat it doesn't take long to wrap around anyway.

Notes

• The advance will wrap around appropriately.
• Holding down button 2 will after 0.4 seconds, autorepeat at 4 times per second.
• The hidden seconds counter is set to zero whenever the minutes are advanced. To synchronise the time to a reference clock, set the minutes to the current minute. When the minute ticks over on the reference clock, click once on button 2.
• If the mode is not at normal display, it will revert to normal display after 64 seconds of button inactivity.

By contrast, the clock chips from National Semiconductor used a multi-button scheme. On the MM5316, I count: Fast set, Slow set, Seconds display, Alarm display, Sleep display, Alarm off, and Snooze. Fast set and slow set require estimating how close one is to the target to change buttons instead of setting the relevant counter directly. A bedside clock has room for more buttons, but I feel that there are too many buttons.

Here is a more elaborate example where the clock stores the date also. Let's put aside how or when the date is displayed. It could be alternately with the time as some old clock chips did, or a separate display:

Here is another elaborate example for an alarm clock radio with sleep timer which is turned on by an alarm match, or by manually setting to non-zero. Notice that button 2 has a function in normal display mode:

The alarm is actually the radio, whose power is switched by the sleep output, doing double duty as an alarm and a sleep radio. Notice that we arrange it so that the display also indicates which mode it is in, to avoid having more indicators. This is not so important when the display technology can provide indicators, like LCDs, and LED raster displays.

The implementation is quite straightforward. It consists of a switch statement for both buttons, and a switch statement for button 2:

static void switchaction()
{
        switch (swstate & B_BOTH) {
        case B_0:
                mode++;
                if (mode >= END_MODE)
                        mode = NORMAL_MODE;
                break;
        case B_1:       // also cancels alarm in normal mode
                switch (mode) {
                case NORMAL_MODE:
                        if (sleep_minutes > 0)
                                sleep_zero();
                        return;
                case TIME_HOURS:
                        mains_time.hours++;
                        if (mains_time.hours >= 24)
                                mains_time.hours = 0;
                        break;
                case TIME_MINS:
                        mains_time.seconds = 0;
                        mains_time.minutes++;
                        if (mains_time.minutes >= 60)
                                mains_time.minutes = 0;
                        break;
                case ALARM_SET:
                        alarm_time.on = !alarm_time.on;
                        break;
                case ALARM_HOURS:
                        alarm_time.hours++;
                        if (alarm_time.hours >= 24)
                                alarm_time.hours = 0;
                        break;
                case ALARM_MINS:
                        alarm_time.minutes++;
                        if (alarm_time.minutes >= 60)
                                alarm_time.minutes = 0;
                        break;
                case SLEEP_MINS:
                        if (sleep_minutes == 0)
                                sleep_on();
                        else
                                sleep_sub1();
                        break;
                }
                display_update();
                alarm_check();
                break;
        }
        if (mode == NORMAL_MODE)
                button_timeout = 0;
        else
                button_timeout = BUTTON_TIMEOUT;
}

The mode-set user interface is not suitable for all devices. It's best for those where setting is rarely done. Its greatest advantages are that it only requires two buttons and the model is easy to understand. It's not so good when multiple settings have to be adjusted around the same time. For example you wouldn't want it for a DSO panel.