SPIM — Serial peripheral interface master with EasyDMA

The SPI master can communicate with multiple slaves using individual chip select signals for each of the slave devices attached to a bus.

Listed here are the main features for the SPIM

Figure 1. SPIM — SPI master with EasyDMA

The SPIM does not implement support for chip select directly. Therefore, the CPU must use available GPIOs to select the correct slave and control this independently of the SPI master. The SPIM supports SPI modes 0 through 3. The CONFIG register allows setting CPOL and CPHA appropriately.

Table 1. SPI modes
Mode Clock polarity Clock phase
  CPOL CPHA
SPI_MODE0 0 (Active High) 0 (Leading)
SPI_MODE1 0 (Active High) 1 (Trailing)
SPI_MODE2 1 (Active Low) 0 (Leading)
SPI_MODE3 1 (Active Low) 1 (Trailing)

Shared resources

The SPI shares registers and other resources with other peripherals that have the same ID as the SPI. Therefore, the user must disable all peripherals that have the same ID as the SPI before the SPI can be configured and used.

Disabling a peripheral that has the same ID as the SPI will not reset any of the registers that are shared with the SPI. It is therefore important to configure all relevant SPI registers explicitly to secure that it operates correctly.

See the Instantiation table in Instantiation for details on peripherals and their IDs.

EasyDMA

The SPI master implements EasyDMA for reading and writing of data packets from and to the DATA RAM without CPU involvement.

The RXD.PTR and TXD.PTR point to the RXD buffer (receive buffer) and TXD buffer (transmit buffer) respectively, see Figure 1. RXD.MAXCNT and TXD.MAXCNT specify the maximum number of bytes allocated to the buffers.

The SPI master will automatically stop transmitting after TXD.MAXCNT bytes have been transmitted and RXD.MAXCNT bytes have been received. If TXD.MAXCNT is larger than RXD.MAXCNT, the superfluous received bytes will be ignored. If RXD.MAXCNT is larger than TXD.MAXCNT, the remaining transmitted bytes will contain the value defined in the ORC register.

If the RXD.PTR and the TXD.PTR are not pointing to the Data RAM region, an EasyDMA transfer may result in a HardFault or RAM corruption. See Memory for more information about the different memory regions.

The .PTR and .MAXCNT registers are double-buffered. They can be updated and prepared for the next transmission immediately after having received the STARTED event.

The ENDRX/ENDTX event indicate that EasyDMA has finished accessing respectively the RX/TX buffer in RAM. The END event gets generated when both RX and TX are finished accessing the buffers in RAM.

EasyDMA list

EasyDMA supports one list type.

The supported list type is:

  • Array list

EasyDMA array list

The EasyDMA array list can be represented by the data structure ArrayList_type.

For illustration, see the code example below. This data structure includes only a buffer with size equal to Channel.MAXCNT. EasyDMA will use the Channel.MAXCNT register to determine when the buffer is full. Replace 'Channel' by the specific data channel you want to use, for instance 'NRF_SPIM->RXD', 'NRF_SPIM->TXD', 'NRF_TWIM->RXD', etc.

The Channel.MAXCNT register cannot be specified larger than the actual size of the buffer. If Channel.MAXCNT is specified larger than the size of the buffer, the EasyDMA channel may overflow the buffer.

This array list does not provide a mechanism to explicitly specify where the next item in the list is located. Instead, it assumes that the list is organized as a linear array where items are located one after the other in RAM. 


      
  #define BUFFER_SIZE  4
  
  typedef struct ArrayList
  {
    uint8_t buffer[BUFFER_SIZE];
  } ArrayList_type;

  ArrayList_type MyArrayList[3];

  //replace 'Channel' below by the specific data channel you want to use,
  //         for instance 'NRF_SPIM->RXD', 'NRF_TWIM->RXD', etc.
  Channel.MAXCNT = BUFFER_SIZE;
  Channel.PTR = &MyArrayList;
Figure 2. EasyDMA array list

SPI master transaction sequence

An SPI master transaction consists of a sequence started by the START task followed by a number of events, and finally the STOP task.

An SPI master transaction is started by triggering the START task. The ENDTX event will be generated when the transmitter has transmitted all bytes in the TXD buffer as specified in the TXD.MAXCNT register. The ENDRX event will be generated when the receiver has filled the RXD buffer, i.e. received the last possible byte as specified in the RXD.MAXCNT register.

Following a START task, the SPI master will generate an END event when both ENDRX and ENDTX have been generated.

The SPI master is stopped by triggering the STOP task. A STOPPED event is generated when the SPI master has stopped.

If the ENDRX event has not already been generated when the SPI master has come to a stop, the SPI master will generate the ENDRX event explicitly even though the RX buffer is not full.

If the ENDTX event has not already been generated when the SPI master has come to a stop, the SPI master will generate the ENDTX event explicitly even though all bytes in the TXD buffer, as specified in the TXD.MAXCNT register, have not been transmitted.

The SPI master is a synchronous interface, and for every byte that is sent, a different byte will be received at the same time; this is illustrated in Figure 3.

Figure 3. SPI master transaction

Low power

When putting the system in low power and the peripheral is not needed, lowest possible power consumption is achieved by stopping, and then disabling the peripheral.

The STOP task may not be always needed (the peripheral might already be stopped), but if it is sent, software shall wait until the STOPPED event was received as a response before disabling the peripheral through the ENABLE register.

Master mode pin configuration

The SCK, MOSI, and MISO signals associated with the SPI master are mapped to physical pins according to the configuration specified in the PSEL.SCK, PSEL.MOSI, and PSEL.MISO registers respectively.

The PSEL.SCK, PSEL.MOSI, and PSEL.MISO registers and their configurations are only used as long as the SPI master is enabled, and retained only as long as the device is in ON mode. PSEL.SCK, PSEL.MOSI and PSEL.MISO must only be configured when the SPI master is disabled.

To secure correct behavior in the SPI, the pins used by the SPI must be configured in the GPIO peripheral as described in Table 2 prior to enabling the SPI. This configuration must be retained in the GPIO for the selected IOs as long as the SPI is enabled.

Only one peripheral can be assigned to drive a particular GPIO pin at a time. Failing to do so may result in unpredictable behavior.

Table 2. GPIO configuration
SPI master signal SPI master pin Direction Output value Comments
SCK As specified in PSEL.SCK Output Same as CONFIG.CPOL  
MOSI As specified in PSEL.MOSI Output 0  
MISO As specified in PSEL.MISO Input Not applicable  

Registers

Table 3. Instances
Base address Peripheral Instance Description Configuration
0x40003000 SPIM SPIM0

SPI master 0

    
0x40004000 SPIM SPIM1

SPI master 1

    
0x40023000 SPIM SPIM2

SPI master 2

    
Table 4. Register Overview
Register Offset Description
TASKS_START 0x010

Start SPI transaction

  
TASKS_STOP 0x014

Stop SPI transaction

  
TASKS_SUSPEND 0x01C

Suspend SPI transaction

  
TASKS_RESUME 0x020

Resume SPI transaction

  
EVENTS_STOPPED 0x104

SPI transaction has stopped

  
EVENTS_ENDRX 0x110

End of RXD buffer reached

  
EVENTS_END 0x118

End of RXD buffer and TXD buffer reached

  
EVENTS_ENDTX 0x120

End of TXD buffer reached

  
EVENTS_STARTED 0x14C

Transaction started

  
SHORTS 0x200

Shortcut register

  
INTENSET 0x304

Enable interrupt

  
INTENCLR 0x308

Disable interrupt

  
ENABLE 0x500

Enable SPIM

  
PSEL.SCK 0x508

Pin select for SCK

  
PSEL.MOSI 0x50C

Pin select for MOSI signal

  
PSEL.MISO 0x510

Pin select for MISO signal

  
FREQUENCY 0x524

SPI frequency. Accuracy depends on the HFCLK source selected.

  
RXD.PTR 0x534

Data pointer

  
RXD.MAXCNT 0x538

Maximum number of bytes in receive buffer

  
RXD.AMOUNT 0x53C

Number of bytes transferred in the last transaction

  
RXD.LIST 0x540

EasyDMA list type

  
TXD.PTR 0x544

Data pointer

  
TXD.MAXCNT 0x548

Maximum number of bytes in transmit buffer

  
TXD.AMOUNT 0x54C

Number of bytes transferred in the last transaction

  
TXD.LIST 0x550

EasyDMA list type

  
CONFIG 0x554

Configuration register

  
ORC 0x5C0

Over-read character. Character clocked out in case and over-read of the TXD buffer.

  

SHORTS

Address offset: 0x200

Shortcut register

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                             A                                  
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

END_START

    

Shortcut between END event and START task

See EVENTS_END and TASKS_START

     

Disabled

0

Disable shortcut

     

Enabled

1

Enable shortcut

 

INTENSET

Address offset: 0x304

Enable interrupt

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                         E                     D   C   B     A  
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

STOPPED

    

Write '1' to Enable interrupt for STOPPED event

See EVENTS_STOPPED

     

Set

1

Enable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

B RW

ENDRX

    

Write '1' to Enable interrupt for ENDRX event

See EVENTS_ENDRX

     

Set

1

Enable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

C RW

END

    

Write '1' to Enable interrupt for END event

See EVENTS_END

     

Set

1

Enable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

D RW

ENDTX

    

Write '1' to Enable interrupt for ENDTX event

See EVENTS_ENDTX

     

Set

1

Enable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

E RW

STARTED

    

Write '1' to Enable interrupt for STARTED event

See EVENTS_STARTED

     

Set

1

Enable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

 

INTENCLR

Address offset: 0x308

Disable interrupt

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                         E                     D   C   B     A  
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

STOPPED

    

Write '1' to Disable interrupt for STOPPED event

See EVENTS_STOPPED

     

Clear

1

Disable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

B RW

ENDRX

    

Write '1' to Disable interrupt for ENDRX event

See EVENTS_ENDRX

     

Clear

1

Disable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

C RW

END

    

Write '1' to Disable interrupt for END event

See EVENTS_END

     

Clear

1

Disable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

D RW

ENDTX

    

Write '1' to Disable interrupt for ENDTX event

See EVENTS_ENDTX

     

Clear

1

Disable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

E RW

STARTED

    

Write '1' to Disable interrupt for STARTED event

See EVENTS_STARTED

     

Clear

1

Disable

     

Disabled

0

Read: Disabled

     

Enabled

1

Read: Enabled

 

ENABLE

Address offset: 0x500

Enable SPIM

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                         A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

ENABLE

    

Enable or disable SPIM

     

Disabled

0

Disable SPIM

     

Enabled

7

Enable SPIM

 

PSEL.SCK

Address offset: 0x508

Pin select for SCK

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id C                                                   A A A A A
Reset 0xFFFFFFFF 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Id RW Field Value Id Value Description
A RW

PIN

  

[0..31]

Pin number

C RW

CONNECT

    

Connection

     

Disconnected

1

Disconnect

     

Connected

0

Connect

 

PSEL.MOSI

Address offset: 0x50C

Pin select for MOSI signal

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id C                                                   A A A A A
Reset 0xFFFFFFFF 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Id RW Field Value Id Value Description
A RW

PIN

  

[0..31]

Pin number

C RW

CONNECT

    

Connection

     

Disconnected

1

Disconnect

     

Connected

0

Connect

 

PSEL.MISO

Address offset: 0x510

Pin select for MISO signal

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id C                                                   A A A A A
Reset 0xFFFFFFFF 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Id RW Field Value Id Value Description
A RW

PIN

  

[0..31]

Pin number

C RW

CONNECT

    

Connection

     

Disconnected

1

Disconnect

     

Connected

0

Connect

 

FREQUENCY

Address offset: 0x524

SPI frequency. Accuracy depends on the HFCLK source selected.

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A
Reset 0x04000000 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

FREQUENCY

    

SPI master data rate

     

K125

0x02000000

125 kbps

     

K250

0x04000000

250 kbps

     

K500

0x08000000

500 kbps

     

M1

0x10000000

1 Mbps

     

M2

0x20000000

2 Mbps

     

M4

0x40000000

4 Mbps

     

M8

0x80000000

8 Mbps

 

RXD.PTR

Address offset: 0x534

Data pointer

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

PTR

    

Data pointer

 

RXD.MAXCNT

Address offset: 0x538

Maximum number of bytes in receive buffer

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                 A A A A A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

MAXCNT

    

Maximum number of bytes in receive buffer

 

RXD.AMOUNT

Address offset: 0x53C

Number of bytes transferred in the last transaction

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                 A A A A A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A R

AMOUNT

    

Number of bytes transferred in the last transaction

 

RXD.LIST

Address offset: 0x540

EasyDMA list type

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                           A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

LIST

    

List type

     

Disabled

0

Disable EasyDMA list

     

ArrayList

1

Use array list

 

TXD.PTR

Address offset: 0x544

Data pointer

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

PTR

    

Data pointer

 

TXD.MAXCNT

Address offset: 0x548

Maximum number of bytes in transmit buffer

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                 A A A A A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

MAXCNT

    

Maximum number of bytes in transmit buffer

 

TXD.AMOUNT

Address offset: 0x54C

Number of bytes transferred in the last transaction

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                 A A A A A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A R

AMOUNT

    

Number of bytes transferred in the last transaction

 

TXD.LIST

Address offset: 0x550

EasyDMA list type

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                           A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

LIST

    

List type

     

Disabled

0

Disable EasyDMA list

     

ArrayList

1

Use array list

 

CONFIG

Address offset: 0x554

Configuration register

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                           C B A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

ORDER

    

Bit order

     

MsbFirst

0

Most significant bit shifted out first

     

LsbFirst

1

Least significant bit shifted out first

B RW

CPHA

    

Serial clock (SCK) phase

     

Leading

0

Sample on leading edge of clock, shift serial data on trailing edge

     

Trailing

1

Sample on trailing edge of clock, shift serial data on leading edge

C RW

CPOL

    

Serial clock (SCK) polarity

     

ActiveHigh

0

Active high

     

ActiveLow

1

Active low

 

ORC

Address offset: 0x5C0

Over-read character. Character clocked out in case and over-read of the TXD buffer.

Bit number 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Id                                                 A A A A A A A A
Reset 0x00000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Id RW Field Value Id Value Description
A RW

ORC

    

Over-read character. Character clocked out in case and over-read of the TXD buffer.

 

Electrical specification

SPIM master interface electrical specifications

Symbol Description Min. Typ. Max. Units
fSPIM

Bit rates for SPIM1

 82 Mbps
ISPIM,2Mbps

Run current for SPIM, 2 Mbps

 50 µA
ISPIM,8Mbps

Run current for SPIM, 8 Mbps

 50 µA
ISPIM,IDLE

Idle current for SPIM (STARTed, no CSN activity)

 1 µA
tSPIM,START

Time from START task to transmission started

 .. .. .. µs

Serial Peripheral Interface Master (SPIM) timing specifications

Symbol Description Min. Typ. Max. Units
tSPIM,CSCK

SCK period

 .. .. .. ns
tSPIM,RSCK,LD

SCK rise time, standard drivea

 tRF,25pF  
tSPIM,RSCK,HD

SCK rise time, high drivea

 tHRF,25pF  
tSPIM,FSCK,LD

SCK fall time, standard drivea

 tRF,25pF  
tSPIM,FSCK,HD

SCK fall time, high drivea

 tHRF,25pF  
tSPIM,WHSCK

SCK high timea

 (0.5*tCSCK) – tRSCK  
tSPIM,WLSCK

SCK low timea

 (0.5*tCSCK) – tFSCK  
tSPIM,SUMI

MISO to CLK edge setup time

 19 ns
tSPIM,HMI

CLK edge to MISO hold time

 18 ns
tSPIM,VMO

CLK edge to MOSI valid

 59 ns
tSPIM,HMO

MOSI hold time after CLK edge

 20 ns
Figure 4. SPIM timing diagram

1 High bit rates may require GPIOs to be set as High Drive, see GPIO chapter for more details.
2 The actual maximum data rate depends on the slave's CLK to MISO and MOSI setup and hold timings.
a At 25pF load, including GPIO pin capacitance, see GPIO spec.
Documentation feedback | Developer Zone | Updated 2021-11-08