Hi, HCS12x datasheet is not very clear on this. Is the CPU clocked by the CPU Clock or Core Clock? Manual refers that CPU Cycle equal Bus Cycle. Thanks

[zegeyealemu]

Hi, HCS12x datasheet is not very clear on this. Is the CPU clocked by the CPU Clock or Core Clock? Manual refers that CPU Cycle equal Bus Cycle. Thanks

[zegeyealemu]

Hi Iggi,

Thank you for your additional response.

Just to make sure we are on the same page: You are saying that:

For the 16-bit HCS12x device

The Bus Clock Frequency = (1/2) the CPU (Core) Frequency.

I am good with this.

That said, logically, CPU Cycle implies a single Period of the CPU Frequency. Here, CPU cycle is re-defined to mean Instruction Cycle and Reference Manual does not have this info.

Thanks again for your continuous support.

Best regards,

Ziggy

[zegeyealemu]

Thanks Iggi. That was what I thought initially (Bus Freq = Cpu Freq/2) also but it might not be right.

Reference manual I was referring to is the MC9212XEP100 Reference Manual. If you go to the section titled 'System Clocks Generator" you will find the following excerpt on the 2nd paragraph.

" The Bus Clock is used to generate the clock visible at the ECLK pin. The Core Clock signal is the clock for the CPU. The Core Clock is twice the Bus Clock. But note that a CPU cycle corresponds to one Bus Clock"

The last sentence above starts with "But note that " inferring that the Core Clock is not the same as the CPU Clock, which COULD mean that the Bus Frequency is EQUAL to CPU frequency.

Question is is the Bus Clock Freq equal to CPU Freq or is it equal to (CPU Freq/2)?

Thanks again and best regards,

Ziggy

[iggi]

Here is some word game. From the cited sentence, we see that Core Clock does not equal to CPU cycle, which is true.

Bus clock is half of the core clock and the core clock is the clock for the CPU as stated in the manual.

The CPU cycle is something different from a clock and it's related to instruction execution.

The instruction execution time is usually represented in (CPU) cycles, not in core clock time. These cycles corresponds to bus clock.

And so, generally, we use term 'bus clocks' to determine the number of cycles for an instruction. For example, NOP instruction takes one bus clock to execute.

regards,

iggi

[iggi]

Hi,

what exact datasheet you mean? What is the device family?

Do you refer to CPUS12XV2 reference manual?

The CPU clock and Core Clock means the same. CPU is the Core.

The bus clock differs and usually it is half the core clock -> fCPU = 2 x fBUS.

Regards,

iggi