Common tick accessible from both ARM and PRU

Satoshi TOMINAGA

Hi All,

I created the feedback control system using DMTimer and its interrupt to kernel module on BeagleBone Black.
But the jitter between interrupt to get system tick is exceed +- 10 usec, average latency is 5 usec, it is not enough considering accuracy of the external device control.

So, I'm planning to use PRU to get tick with low / fixed latency.
Please find the attached PRU for the plan.

External signal is connected to GPIO which assigned to PRU.

The GPIO triggers periodical counter programmed on PRU.
When the counter is expired, system tick is obtained and triggers interrupt to ARM core, the obtained tick is sent by shared RAM.
On ARM, the feedback is calculated using the tick and in kernel module linked with user space application which has the common tick table.

So, the comparable tick accessible from both ARM and PRU is needed.
Are there any ticks for this purpose ?
If yes, please let me know how to access it.

Regards,
TOMINAGA Satoshi

feedback_loop_using_PRU.pdf

over 9 years ago

0 Biser Gatchev-XID over 9 years ago

TI__Guru**** 393215 points

Hi,

What Linux version are you using?

0 Satoshi TOMINAGA over 9 years ago in reply to Biser Gatchev-XID

Intellectual 850 points

Hi Biser,

I'm currently using TI SDK v02.00.00.00.
I'm planning to change to the SDK to v02.00.01.07, PRU support is included.

Regards,
TOMINAGA Satoshi

0 melissaw over 9 years ago in reply to Satoshi TOMINAGA

TI__Genius 15910 points

Hi,

To minimize latency on the PRU side, I would recommend looking at the PRU-ICSS IEP Timer. Both the PRU and ARM would be able to access the associated registers.

The PRU software package has several examples using this timer:

examples/<device>/PRU_IEP
pru_cape/pru_fw/PRU_HDQ_TempSensor<0/1>

Regards,

Melissa

0 Satoshi TOMINAGA over 9 years ago in reply to melissaw

Intellectual 850 points

Hi Melissa,

Thank you for your support.
I'll try to use IEP Timer for common tick.

Are there any examples how to access IEP Timer from ARM?

Regards,
TOMINAGA Satoshi

0 melissaw over 9 years ago in reply to Satoshi TOMINAGA

TI__Genius 15910 points

None that I am aware of.

Regards,
Melissa

0 Satoshi TOMINAGA over 9 years ago in reply to melissaw

Intellectual 850 points

Hi Melissa,

Is it impossible to access IEP clock directory from ARM?

or it is possible to access directly, but no examples exist ?

Regards,

TOMINAGA Satoshi

0 melissaw over 9 years ago in reply to Satoshi TOMINAGA

TI__Genius 15910 points

It is possible to access the IEP timer registers directly from the ARM, just no examples exist.

Regards,

Melissa

0 Satoshi TOMINAGA over 9 years ago in reply to melissaw

Intellectual 850 points

Hi Melissa,

Thank you for your support.

In the latest AM335x Sitara™ Processors Technical Reference Manual
Literature Number: SPRUH73M
October 2011–Revised January 2016

IEP Timer Count Register is described in the following section.
4.5.4.4 IEP_TMR_CNT Register (offset = Ch) [reset = 0h]

The offset 0x0C is the same as the following description in pur_iep.h of PRU example of SDK v02.00.01.07.

/* PRU_IEP_TMR_CNT register bit field */
union {
volatile uint32_t TMR_CNT;

volatile struct{
unsigned COUNT : 32; // 31:0
} TMR_CNT_bit;
} ; // 0xC

So, I think this register mapping is the PRU side.
I can't find the register mapping for the ARM side.

Are there any information in documents or kernel source codes?

Regards,
TOMINAGA Satoshi

0 Satoshi TOMINAGA over 9 years ago in reply to melissaw

Intellectual 850 points

Hi Melissa,

Unfortunately, I can't find any documents to access IEP timer registers directory from ARM.
If it is possible as you said, please let me know the information.

Regards,
TOMINAGA Satoshi

0 SaS over 9 years ago in reply to Satoshi TOMINAGA

TI__Intellectual 2060 points

Hi
TRM Table 2-4 L4 Fast Peripheral Memory Map shows PRU_ICSS is mapped at 0x4A30 0000.
See also 4.3.2 Global Memory Map.

Sas

0 Satoshi TOMINAGA over 9 years ago in reply to SaS

Intellectual 850 points

Hi Sas,

Thank you for your support.
I find the description in the document.

I also find the IEP address in 4.3.2 Global Memory Map as
0x0002_E000 IEP

And by the header file of example-applications/pru-icss-4.0.1/include/am335x/pru_iep.h
I find the address offset of TMR_CNT is 0x0C.

So, I think the address of IEP timer from ARM must be:
0x4A30 0000 + 0x0002 E000 + 0x0000 000C = 0x4A32 E00C

Is this right?

And, I've never write the linux application which access memory mapped register directory.
Are there any sample for the access ? Not IEP registers are accepted, I want to know the
procedure to access.
If there are special process to access the IEP registers, which is also accessed from PRU,
I also want to know.

Regards,
TOMINAGA Satoshi

0 SaS over 9 years ago in reply to Satoshi TOMINAGA

TI__Intellectual 2060 points

Yes. ARM can access IEP_TMR_CNT reg at 0x4A30 0000 + 0x0002 E000 + 0x0000 000C = 0x4A32 E00C.

I don't know Linux programs handling IEP, but
pru_cape/pru_demo would help you which access pru_icss RAM from ARM.

Sas

0 Satoshi TOMINAGA over 9 years ago in reply to SaS

Intellectual 850 points

Hi Sas,

Thank you for your information.

I'll try to access the TIMER register.

Regards,

TOMINAGA Satoshi

0 Satoshi TOMINAGA over 9 years ago in reply to Satoshi TOMINAGA

Intellectual 850 points

Hi Sas,

Using mmap access to specified address, I succeeded to access PRU IEP Timer !

I made modification to example-applications/pru-ices-4.0.1/examples/am335x/PRU_IEP/PRU_IEP.c.

Do not disable IEP Timer, do not _halt() but while(1) at the end of main().

With this modification, read value with mmap from ARM increased around the rate of 200000000 per second,

which is iep_clk frequency.

I'll try to modify the code to add communication between PRU and ARM.

Thank you for your support.

Regards,

TOMINAGA Satoshi

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Common tick accessible from both ARM and PRU