It's purely educational, that's why I'm not using MAX7219.Can I use output pins instead of transistors? But how can an output pin source or sink current for all 8 digit leds simultaneously?
A In the alternate method, each segment is ON 1/8 of the time. So basically you have to refresh twice as often to prevent flicker.
In a 4-digit multiplexed display unit, for how long time the cc-pin of a digit could be hold at Logic-Low state so that the message at other digit positions does not appear to be absent (aka flickering). To say in other way, what is the frequency of the walking-0 signal across the cc-pins of the display unit. The famous 8279 chip was using a 100 kHz scanning circuit to refresh the 16-digit multiplexed display unit. At this standard, the refresh time for a cc-pin is maximum: 1/100^3 = 100 us.