Why? Is SPI a problem at high speed?

[jongjupark]

Dear community,

**Question**
Is SPI a problem at high speed?
 

**description**

1. Chip : MCIMX6Q5EYM10AD

2. Kernel Version : L3.14.28

I have tested 20Mhz from 10Mhz with SPI Master Mode between iMX6Q and SPI device.

I simply tested the memory of the SPI device to read it.
Up to 10Mhz ~ 14Mhz worked normally.
However, since 15Mhz, there is a problem with the data.

▣ SPI communication result according to frequency

MHz	Result	Read Data
10	OK	0x92520001
11	OK	0x92520001
12	OK	0x92520001
13	OK	0x92520001
14	OK	0x92520001
15	FAIL	0xc9290000
16	FAIL	0xc9290000
17	FAIL	0xc9290000
18	FAIL	0xc9290000
19	FAIL	0xc9290000
20	FAIL	0xc9290000

The difference between the normal data (0x92520001) and the abnormal data (0xc9290000)
Moving the least significant bit of normal data forward is exactly like abnormal data. This seems to be a bug in the SPI driver.

▣ Datas comparison

Hex	Bit
0x92520001	1001 0010 0101 0010 0000 0000 0000 0001 <--- least significant bit
0xc9290000	1100 1001 0010 1001 0000 0000 0000 0000

1001 0010 0101 0010 0000 0000 0000 0001 (0x92520001)

11001 0010 0101 0010 0000 0000 0000 000

1100 1001 0010 1001 0000 0000 0000 0000 (0xc9290000)

[igorpadykov]

Hi jongju

one can consult with datasheet connected device if it supports 20Mhz

and check with oscilloscope timings using sect.4.12.2.1 ECSPI Master Mode Timing

i.MX6DQ Datasheet

http://www.nxp.com/docs/en/data-sheet/IMX6DQCEC.pdf

Best regards
igor
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[jongjupark]

The waveform below is the value I checked through the oscilloscope.
In conclusion, the waveform datas of the oscilloscope is ok.

However, there is a problem with the value read from the SPI driver.

[waveform measurement 1] 10Mhz SPI total waveform
- Yellow: Chip Select
- Green : MOSI
- Blue : MISO
- Red : Clock

[waveform measurement 2] 10Mhz SPI detailed waveform
- Yellow: Chip Select
- Green : MOSI 
- Blue : MISO
- Red : Clock
The waveform below shows that "0x92 0x52 0x00 0x01" data is normally read.

[waveform measurement 3] 20Mhz SPI total waveform 
- Yellow: Chip Select
- Green : MOSI 
- Blue : MISO
- Red : Clock

[waveform measurement 4] 20Mhz SPI detailed waveform
- Yellow: Chip Select
- Green : MOSI 
- Blue : MISO
- Red : Clock

The waveform below shows that "0x92 0x52 0x00 0x01" data is normally read.
(At 20Mhz, the right end was cut off on the screen. However, 0x92 was read normally.)