Lowering PIC Clock Frequency

Question

How do I lower the clock frequency of a PIC18F2550 and recalculate the peripheral dividers?

I'm trying to make it run cooler. Currently, it runs at 12MIPs/48mhz (USB-CDC-ACM).

Answer 1

If you're currently running at 48 MHz, you're using an external crystal. You can decrease the frequency by changing bits 2:0 in the PLLDIV register, but we'd need to know the external crystal frequency and what peripherals you want to adjust. Is it just the USB subsystem you need to use?

In response to your comment:

Looks like you can get it down to 5 MHz with no hardware changes. Take a look at Table 2-3 the datasheet. Switch clock mode to disable the PLL and set CPUDIV1:0 to 11 and you'll get a 4x divider down to 5 MHZ.

Answer 2

Depending on your application, you may want to try putting the PIC to sleep and using an interrupt with an RC circuit to wake it up periodically.

Answer 3

That's a weird place to have the board get hot. It could definitely be warm at the other end, near R4, T1, and the TX LED, but there's nothing happening on the end which you say is warm! (Schematic, layout)

According to the datasheet (p. 363), the power dissipated in your PIC should be:

Pdis = VDD x {IDD – Σ IOH} + Σ {(VDD – VOH) x IOH} + Σ(VOL x IOL)

This uses the Power = Voltage * Current equation to say that the total power dissipated is the sum of the power consumed by the chip's operation (not including driven outputs) plus any power dissipation on the current sourcing transistors, plus any power dissipation on the current sinking transistors. See page 375 for data on output voltages and currents. However, it looks like there should be only minimal power dissipation according to this equation.

Three things I would look at:
- The next page has a voltage/max frequency graph. It shows that you're only allowed to operate at 48MHz when you're running above 4.2V. Is this the case when you're operating off of your ICSP header? Running out of spec here might cause issues by not switching the transistors fast enough, and developing shoot-through currents.
- That micro includes an onboard 3.3V LDO for the USB. If you're drawing any current off of this pin, you will dissipate power in the micro. If you're not communicating over USB all the time, you can disable this regulator with the VREGEN configuration bit in the CONFIG2L register.
- You got it hot once, it may now have a weak internal short. Try a different microcontroller or board. This is unlikely, but possible.