16.3 Accessing 16-bit RegistersThe TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via the 8-bit data bus. The 16-bit register must be byte accessed using two read or write operations. Each 16-bit timer has a single 8-bit register for temporary storing of the high byte of the 16-bit access. The same temporary register is shared between all 16-bit registers within each 16-bit timer. Accessing the low byte triggers the 16-bit read or write operation. When the low byte of a 16-bit register is written by the CPU, the high byte stored in the temporary register, and the low byte written are both copied into the 16-bit register in the same clock cycle. When the low byte of a 16-bit register is read by the CPU, the high byte of the 16-bit register is copied into the temporary register in the same clock cycle asthe low byte is read.
We've set Timer1 to interrupt on an overflow, and let's assume we're using an ATmega328 with a 16MHz clock. Since Timer1 is 16 bits, it can hold a maximum value of (2^16 - 1), or 65535. At 16MHz, we'll go through one clock cycle every 1/(16*10^6) seconds, or 6.25e-8 s. That means 65535 timer counts will elapse in (65535 * 6.25e-8s) and our ISR will trigger in, oh… about 0.0041 seconds. Then again and again, every four thousandths of a second after that. Oops. At this rate, we probably won't even be able to detect blinking. If anything, we've created an extremely fast PWM signal for the LED that's running at a 50% duty cycle, so it may appear to be constantly on but dimmer than normal. An experiment like this shows the amazing power of microprocessors - even an inexpensive 8-bit chip can process information far faster than we can detect.