Electronic – Calculating Timer Tick STM32F4

As Tut was first to suggest, the observed error arrises from the fact that the total number of cycles in these dividers is one more than the value loaded into the corresponding register.

You took this into account in the prescaler register, but not in the period register.

When you apportion the net division equitably between the two stages, each has a moderately large value and so the effect of "off-by-one" is small.

But when you apportion the division unequally such that one value is only 28, the difference between 29 counts (28+1) vs the correct 28 (27+1) is responsible for the 1-part-in-28 error you see.

Unless you have made any changes to the oscillator settings with the OSCCON register it will default to a prescaler of /8 to give 1MHz. If that is true, then the \$F_{CY}\$ will be 256000Hz.

Your prescaler is set to 256, so the clock tick will be at \$\frac{256000}{256}=1000Hz\$

You think you're setting the overflow value to 65535, but you're not. You're actually setting the current timer's high byte to 0xFFFF (which, since it's an 8 bit register is actually 0xFF), and the timer's low byte to 0x00, so the current timer count is 0xFF00 in total. It ticks up, and when it loops from 0xFFFF to 0x0000 it triggers an interrupt. That's 256 ticks away.

So at 1000Hz divided by 256 ticks, that's \$\frac{1000}{256}=3.90625Hz.\$

Which is almost 4 times per second.

So to get exactly one second, with a 1000Hz tick, you need 1000 ticks. So you would set your timer values to 65536 minus 1000, or 64536, which is 0xFC18, so your TIMERH would be 0xFC and your TIMERL would be 0x18.