The Arduino has a way to access the port registers (official documentation here), it allows lower-level and fast manipulation of the i/o pins on the micro-controller. This pertains to the ATmega8 and ATmega168. I did my testing with a Diecimila (ATmega168).

Now you may be wondering why I am making a post about this, well I ran into a interesting scenario while trying to use the registers to read if a pin was high or low. Now it is covered in the official documentation but I thought it would be nice to create a chart and elaborate on it some more.

When I was reading the output on digital pins 0-7 I had no issues, since I could define the ping with a variable and pass that variable to the bitRead() function and get what I needed. What happened is if I had a variables for say pin 8 and then passed that variable to the bitRead() and I would get nothing back.

Turns out what I did not know is this pings 0-7 map directly with the bits used to access the lower level of the chip.

As you can see its gos in descending order from right to left. So if you want to set pins 3&4 for output on DDRD it would be DDRD = B00011000, since you either have on (1) or off (0).

Now back to my issue, I was passing the variable for pin 8 which as you can see there is no 8 on the DDRB, so I had to set it to 0 without using the variable. Now not knowing this did cause me about an hour of troubleshooting. But now that I learned this its much easier.

WARNING I am not 100% sure if DDRB can go past 4 bits or if it has to be 4, so be CAREFUL!

Example say you wanted to set ping 8 & 9 to output. DDRB = B0011. Now on the official documents it only has DDRB with four bits. So you might only be able to set pin 8-11. So read up on it first, before you try to go higher, I only used bitRead() on mine.

If you want to read if the output on pin 9 is high or low just use “bitRead(PORTD,1)”.

I hope this information is useful to someone. This information is in the Arduino official documentation, the issue I had is it was not painfully obvious to me.