^13 - The Counter Port

The counter port ^13,0 is a read only value of the low byte of the internal counter for the device. As this value is constantly changing it is useful as a random number generator for light chases etc.
It updates around once every 300ms.

As this is counting up at a predictable rate it isn’t truly random and may create a repetitive pattern.

Syntax

LDA ^13,0

Examples

Example task where counter port recalls a random colour.

Task1()
{
LDA ^13,0        // Get the counter port
CMP #25          // Compare to 25
BCC Colour1       // If less than branch
CMP #50          // Compare
BRZ Colour2       // if less than branch
CMP #50          // Compare
BRZ Colour3       // if less than branch
CMP #70          // Compare
BRZ Colour4       // if less than branch
CMP #90          // Compare
BRZ Colour5       // if less than branch
CMP #110         // Compare
BRZ Colour6       // if less than branch
CMP #130         // Compare
BRZ Colour7       // if less than branch
CMP #150         // Compare
BRZ Colour8       // if less than branch
...
}