topic Re: clock cycle activity for LPC1114 in LPC Microcontrollers

clock cycle activity for LPC1114

heaton_1991 — Tue, 27 Nov 2018 23:44:35 GMT

I am now working with the LPC1114 which utilizes the ARM CORTEX M0 architecture. I have one question about the instruction set summary of the ARMv6M Thumb instruction set. I want to know what the processor does during each single clock cycle for each instruction. For example, if I have the following code where I want to write something to the GPIO0DATA register to change the level of the IO output (Actually toggle PIO0_3)
loop
LDR R0, =(0x50003FFC); GPIO0DATA Base + 0x3FFC, address 0x5000 3FFC
LDR R1, [R0];
MOVS R2, #(1<<3);
; Store the value of R1 into GPIO0DATA
EORS R1, R1, R2;
STR R1, [R0];
B loop
Question 1:
Let's say the first clock cycle is when the chip fetches the LDR R0, =(0x50003FFC) instruction. what the chip does in the following clock cycles? Also if there is any reference that could explain it, that will be really helpful.

Question 2:
I find that the time between PIO0_3 is toggled every 15 cycles. However, based on the instruction set summary, it should be 11 cycles (LDR/STR takes two cycles and MOVS, EORS takes 1 cycle, B takes 3 cycles), anyone knows why? If there is a timing diagram to explain it, that would be great!

Re: clock cycle activity for LPC1114

xiangjun_rong — Wed, 28 Nov 2018 06:02:03 GMT

Hi, Liao,

I suppose that you can modify your code as following to save instruction cycle time.

LDR R0, =(0x50003FFC); GPIO0DATA Base + 0x3FFC, address 0x5000 3FFC
loop:LDR R1, [R0];
MOVS R2, #(1<<3);
; Store the value of R1 into GPIO0DATA
EORS R1, R1, R2;
STR R1, [R0];
B loop

Because the R0 is not changed in the loop, so you can move loop label in the next line to save cycle time.

Question 1:
Let's say the first clock cycle is when the chip fetches the LDR R0, =(0x50003FFC) instruction. what the chip does in the following clock cycles? Also if there is any reference that could explain it, that will be really helpful.

>>>>>>The line LDR R0, =(0x50003FFC) is a macro, it is replaced with two movs instruction. As you know that the Cortex-M0 core instruction is word(32 bits) or half word(16 bits),because the operand 0x50003FFC is 32 bits so it is replaced by two moves instructions.

>>>>>I do not know where you get the cycle time of each instruction, anyway, because of pipeline, maybe the jump instruction B occupies multiple cycles. The Cortex-M0 is ARM IP, I suggest you ask ARM company directly, we have not detailed doc about the core.

Hope it can help you

Xiangjun Rong

Re: clock cycle activity for LPC1114

converse — Wed, 28 Nov 2018 09:11:39 GMT

One reason for code executing more slowly than you expect is that the code is executing out of Flash, which can introduce wait-states.

Re: clock cycle activity for LPC1114

heaton_1991 — Wed, 28 Nov 2018 17:41:40 GMT

Hi, Rong,

You mentioned that "The line LDR R0, =(0x50003FFC) is a macro, it is

replaced with two movs instruction", Could you please explain what the two

movs instructions are?

Thanks,

Haohao.

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Re: clock cycle activity for LPC1114

xiangjun_rong — Thu, 29 Nov 2018 07:34:29 GMT

Hi, Liao,

Anyway, the LDR Rd=constant is a pseudo-instruction, maybe different compiler deal with in different way.

This is the part I find in DUI0801E_armasm_user_guide.pdf

Hope it can help you

XiangJun Rong

6.7 Load immediate values using LDR Rd, =const
The LDR Rd,=const pseudo-instruction generates the most efficient single instruction to load any 32-bit
number.
You can use this pseudo-instruction to generate constants that are out of range of the MOV and MVN
instructions.
The LDR pseudo-instruction generates the most efficient single instruction for the specified immediate
value:
• If the immediate value can be constructed with a single MOV or MVN instruction, the assembler
generates the appropriate instruction.
• If the immediate value cannot be constructed with a single MOV or MVN instruction, the assembler:
— Places the value in a literal pool (a portion of memory embedded in the code to hold constant
values).
— Generates an LDR instruction with a PC-relative address that reads the constant from the literal
pool.
For example:
LDR rn, [pc, #offset to literal pool]
; load register n with one word
; from the address [pc + offset]
You must ensure that there is a literal pool within range of the LDR instruction generated by the
assembler.

Re: clock cycle activity for LPC1114

heaton_1991 — Thu, 29 Nov 2018 16:58:31 GMT

Hi, Rong,

Many thanks for your help! I really appreciate it!

xiangjun.rong <admin@community.nxp.com> 于2018年11月29日周四上午2:35写道：

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