NSS分為內部引腳和外部引腳。
NSS外部引腳可以作為輸入信號或者輸出信號，
輸入信號一般用作硬件方式從機的片選，
而輸出信號一般用於主SPI去片選與之相連的從SPI。
NSS從設備選擇有兩種模式：
1、軟件模式
可以通過設置SPI_CR1寄存器的SSM位來使能這種模式，當它為1時，NSS引腳上的電平由SSI決定。
在這種模式下NSS外部引腳可以用作它用，而內部NSS信號電平可以通過寫SPI_CR1的SSI位來驅動。
2、硬件模式兩種方式：
（1）對於主SPI，NSS可以直接接高電平，對於從SPI，可以直接接低電平。
（2）當STM32F10xxx工作為主SPI，並且NSS輸出已經通過SPI_CR2寄存器的SSOE位使能，
這時主機的NSS講作為輸出信號，引腳信號被拉低，所有NSS引腳與這個主SPI的NSS引腳相連
並配置為硬件NSS的STM32F10xxx SPI設備，將自動變成從SPI設備。
此時兩個的NSS信號線可以接個上拉電阻直連。
按照標准的SPI協議，當SPI被配置為主機模式后，通過SPI對從設備進行操作時，
其NSS應該自動置低，從而選中（使能）從設備；
一旦不對從設備進行操作，NSS立刻置為高。
但是，我在實際調試過程中卻發現：STM32 SPI NSS無法自動實現跳變。
一旦SPI初始化完成並使能SPI，NSS立刻置低，然后保持不變。
這個問題一直無法解決，直到我在ST官方論壇上看到國外有些技術人員也在討論這個問題，
他們得出的結論是：STM32 SPI NSS無法自動跳變。
ST官方技術人員也證實：
STM32 SPI NSS是不會自動置位和復位的。按照官方說法，ST已經將其列入了改進計划。
對於這個問題，可以采用下面的方法解決：
在SPI初始化時，采用NSS soft模式，然后使能NSS輸出功能。
從而將NSS當做GPIO使用，通過軟件set和reset來實現NSS的置位和復位。
通過將NSS配置為GPIO，在通過SPI操作從設備時，就可以通過軟件來選中和釋放從設備了。
雖然比起硬件自動置位要麻煩，但問題畢竟解決了。
Slave select (NSS) pin management
There are two NSS modes:
● Software NSS mode:
this mode is enabled by setting the SSM bit in the SPI_CR1 register (see Figure 209).
In this mode, the external NSS pin is free for other application uses
and the internal NSS signal level is driven by writing to the SSI bit in the SPI_CR1 register.
● Hardware NSS mode: there are two cases:
– NSS output is enabled:
when the STM32F20xxx is operating as a Master and the NSS output is enabled
through the SSOE bit in the SPI_CR2 register, the NSS pin is driven low and all the NSS pins
of devices connected to the Master NSS pin see a low level
and become slaves when they are configured in NSS hardware mode.
When an SPI wants to broadcast a message, it has to pull NSS low to inform all others
that there is now a master for the bus.
If it fails to pull NSS low, this means that there is another master communicating,
and a Hard Fault error occurs.
– NSS output is disabled: the multimaster capability is allowed.
當SPI配置為hard模式后，通過檢測NSS可以實現的是自身主機和從機模式的切換，
而不是大多數人所認為的自動NSS。。。
也就是說：在一個多SPI系統中，STM32 SPI通過NSS檢測，一旦發現系統中無NSS低信號，自己就輸出低，從而成為主機；
當系統中有NSS低信號時（及已經有其它SPI宣布為主機），自己就配置為從機。
所謂的hard模式的NSS，實際就是為了實現多機間通信的。
STM32 SPI NSS discussion
About the the STM32 SPI NSS problem of explore.
The STM32 SPI Reference Manual in the schematic given as follows:
Accordance with the standard SPI protocol, when the SPI is configured for master mode,
the SPI operation from the device, the NSS should be automatically set low,
so as to select (enable) from the device; Once we do not operate from the device, NSS immediately set is high.
However, I was found in the actual process of debugging:
STM32 SPI NSS does not automatically transition.
Once the the SPI initialization is complete make can SPI NSS immediately set low,
and then remain unchanged.
This problem can not be resolved, some foreign technicians are also discussing this issue
until I see the ST official forum, They concluded: the STM32 SPI NSS can not be automatically transition.
The official ST technicians also confirmed:
the STM32 SPI NSS is not automatically set and reset.According to the official statement,
ST has included in the improvement plan.
For this problem, you can use the following solutions:
SPI initialization the the NSS soft mode, and then enable the NSS output function.
NSS NSS set and reset as GPIO, set and reset by software.
The code is as follows:
/ * SPI1 configuration ---------------------------------------------- -------- * / SPI_InitStructure.SPI_Direction = SPI_Direction_1Line_Tx; SPI_InitStructure.SPI_Mode = SPI_Mode_Master; SPI_InitStructure.SPI_DataSize = SPI_DataSize_8b; SPI_InitStructure.SPI_CPOL = SPI_CPOL_Low; SPI_InitStructure.SPI_CPHA = SPI_CPHA_2Edge; SPI_InitStructure.SPI_NSS = SPI_NSS_Soft; / / SPI_InitStructure.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_4; SPI_InitStructure.SPI_FirstBit = SPI_FirstBit_MSB ;/ / SPI_FirstBit_MSB; SPI_InitStructure.SPI_CRCPolynomial = 7; SPI_Init (SPI1, & SPI_InitStructure); / * Enable SPI1.NSS as a GPIO * / SPI_SSOutputCmd (SPI1, ENABLE); / * Configure PA.4 (NSS) ----------------------------------------- --- * / GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4; GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP; GPIO_Init (GPIOA, & GPIO_InitStructure);
Through the NSS configuration the GPIO, from the device through the SPI operation
can be software selected and released from the device.
Although than the hardware automatically set to trouble, but after all solved.
When the SPI is configured as a hard mode can be achieved by detecting the NSS own host and slave mode switch,
rather than automatically NSS most people think. . .
In other words:
a multi-SPI system STM32 SPI NSS testing, once the system NSS low signal output low, thus becoming the host;
NSS low signal system (and other SPI announced host), configured as a slave.
The so-called hard mode NSS is in fact in order to achieve multi-machine communication.
Summary:
The words too literally scary, Manual must be carefully read.
The STM32 SPI NSS regardless of the configuration for soft or hard are not automatically set, but it is required in most applications. As the ST forum RichardE said: "Everything would be done by the peripheral. Fire and forget."
Oh, interesting argument: Fire and forget! ~ ~ ~ ~
The STM32 SPI nss Big Secret
On the NSS, I have a headache for it for a long time, look at the manual to see the program,
watch videos, see post, I have not completely understood it.
Had a few want to solve it, but died, only to abandon their own written notes.
Also so much concern about the lack of blood, angry, resulting toothache, chapped lips.
Simply a pain. At that time, I really think I may never do not understand.
So forget it. The teacher said, let me do first technology, go pursuit of principle,
but then I took over the ADXL345 acceleration sensor with SPI control task
and a combination of a full-duplex SPI instance, was thus slowly mystery surfaced by learning debugging techniques,
and finally understood. SPI is really complicated, but complex and interesting.
I write as a rookie, I understand the STM32 SPI NSS. I hope you will correct me.
The NSS in the end is how it? The answer is the chip select.
Master and during SPI communication from the device, the device has a CS chip select signal, active low,
we usually must be connected to the NSS from the CS.
But just in general so that we generally understand what's going on,
a lot of things about the NSS, there are a lot of things, let me explain.
Look at the input and output mode.
Can be entered for each SPI NSS can also output. So-called input the NSS level signal to itself,
the so-called output the NSS level signal sent to the slave.
Configured as an output, or output, we the SSOE bit of through SPI_CR2 register.
SSOE 1 and SPI is in master mode control, NSS output low is pulled low,
the NSS pin so when the other SPI devices connected to it, inevitably received low,
the chip select , have become from the device. The output of the NSS introduced here
The following describes the input of the NSS.
We all know that the NSS input hardware inputs and software is divided into the control input two modes,
then you start from these two modes, to uncover the veil.
Let me talk about the software mode.
1 for SPI master
set SPI_CR1 register SSM and SSI bit the SSM to 1 to
enable software management. NSS internal and external pin.
At this time, the external pin reserved for him (can be used as GPIO driver from the device's chip select signal).
Internal NSS pin level driven by the register SPI_CRL SSI bit.
SSI bit to 1 in order to enable the NSS level is high.
At this time, can not help but question why the main device's internal NSS level 1?
STM32 manual, to keep the MSTR and SPE bit is 1, that is to keep the host mode,
only NSS received a high level signal, the two in order to maintain the set that is
to keep the SPI for STM32 the host state, the NSS internal input level must be high.
Of course, here in hardware mode as well.
2 SPI slave
The to host own internal NSS high level resolved, then the SPI chip select active low slave NSS have to solve ah.
If the Slave Select the STM32 an SPI, such as host elected SPI1 slave elected SPI2, will have the following operation
The manual says the NSS pin must be connected to a low-level signal before completing the byte transfer.
The SSM bit software mode, you need to set the SPI_CR1 register (software management enabled)
and SSI bit 0 is really the case. SSI must be 0, which is the SPI2 sheet preferably is low, then the chip select successful.
Slave SPI chip, for example, that the ADXL345 acceleration sensor.
Well, we can There are two ways
A method is the CS of the chip is connected to GND,
another method, a GPIO port to output a low level to control the CS Chip Select success.
This GPIO can be any one GPIO port, of course, we mentioned above SPI Host Configuration software mode,
the external NSS pin reserved for him, it is a GPIO, we can also use it.
At this time, we can set the push-pull output is low,
and then follow the machine line is connected to the CS, then you can chip select from the chip.
Say that the hardware mode.
For the host, the NSS can be connected directly to the high, slave, NSS low.
Of course, we mentioned above, when a host SSOE 1, the master work in output mode,
and NSS is pulled low, we want the slave select, as long as the CS is connected to the host NSS CSautomatically pulled low.
This is ST designed STM32 SPI NSS workflow.
The following examples to introduce.
We introduce a STM32 on SPI1 and SPI2 full-duplex communication procedures,
specific procedures, you can add QQ 843538946 asked me to.
Only describes the the SPI configuration program.
/ * SPI1 Config ---------------------------------------------- --------------- * /
SPI_InitStructure.SPI_Direction = SPI_Direction_2Lines_FullDuplex;
SPI_InitStructure.SPI_Mode = SPI_Mode_Master, / / set SPI1-based mode, set the SSI 1
SPI_InitStructure.SPI_DataSize = SPI_DataSize_8b;
SPI_InitStructure.SPI_CPOL = SPI_CPOL_Low;
SPI_InitStructure.SPI_CPHA = SPI_CPHA_2Edge;
SPI_InitStructure.SPI_NSS = SPI_NSS_Soft; / / set the SSM, software management
SPI_InitStructure.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_4;
SPI_InitStructure.SPI_FirstBit = SPI_FirstBit_LSB;
SPI_InitStructure.SPI_CRCPolynomial = 7;
SPI_Init (SPI1, & SPI_InitStructure);
/ * SPI2 Config ---------------------------------------------- --------------- * /
SPI_InitStructure.SPI_Mode = SPI_Mode_Slave ;/ / set here SPI2 mode SSI
/ / Because SPI2 and SPI1 use the same SPI_InitStructure, so SSM bit is 1
SPI_Init (SPI2, & SPI_InitStructure);
For SPI2 the configuration, and SPI1 with the same structure, just mode should be changed
and SSI on the line, such as software enable, as well as the timing of what do not change.
SuchSPI1 and SPI2 configuration to.
Later you can transfer data.