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Communicating as master and slave via SPI

This example code shows how to interface two RP2040 microcontrollers to each other using SPI.

Wiring information

Function Master (RP2040) Slave (RP2040) Master (Pico) Slave (Pico)

MOSI

DO0

DI0

25

21

SCLK

SCK0

SCK0

24

24

GND

GND

GND

23

23

CS

CS0

CS0

22

22

MISO

DI0

DO0

21

25

spi master slave bb
Figure 1. Wiring Diagram for SPI Master and Slave.

At least one of the boards should be powered, and will share power to the other.

If the master is not connected properly to a slave, the master will report reading all zeroes.

If the slave is not connected properly to a master, it will initialize but never transmit nor receive, because it’s waiting for clock signal from the master.

Outputs

Both master and slave boards will give output to stdio.

With master and slave properly connected, the master should output something like this:

SPI master example
SPI master says: The following buffer will be written to MOSI endlessly:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f
40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f
50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f
60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f
70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f
80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f
90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f
a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af
b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf
c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf
d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df
e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef
f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff
SPI master says: read page 0 from the MISO line:
ff fe fd fc fb fa f9 f8 f7 f6 f5 f4 f3 f2 f1 f0
ef ee ed ec eb ea e9 e8 e7 e6 e5 e4 e3 e2 e1 e0
df de dd dc db da d9 d8 d7 d6 d5 d4 d3 d2 d1 d0
cf ce cd cc cb ca c9 c8 c7 c6 c5 c4 c3 c2 c1 c0
bf be bd bc bb ba b9 b8 b7 b6 b5 b4 b3 b2 b1 b0
af ae ad ac ab aa a9 a8 a7 a6 a5 a4 a3 a2 a1 a0
9f 9e 9d 9c 9b 9a 99 98 97 96 95 94 93 92 91 90
8f 8e 8d 8c 8b 8a 89 88 87 86 85 84 83 82 81 80
7f 7e 7d 7c 7b 7a 79 78 77 76 75 74 73 72 71 70
6f 6e 6d 6c 6b 6a 69 68 67 66 65 64 63 62 61 60
5f 5e 5d 5c 5b 5a 59 58 57 56 55 54 53 52 51 50
4f 4e 4d 4c 4b 4a 49 48 47 46 45 44 43 42 41 40
3f 3e 3d 3c 3b 3a 39 38 37 36 35 34 33 32 31 30
2f 2e 2d 2c 2b 2a 29 28 27 26 25 24 23 22 21 20
1f 1e 1d 1c 1b 1a 19 18 17 16 15 14 13 12 11 10
0f 0e 0d 0c 0b 0a 09 08 07 06 05 04 03 02 01 00

The slave should output something like this:

SPI slave example
SPI slave says: When reading from MOSI, the following buffer will be written to MISO:
ff fe fd fc fb fa f9 f8 f7 f6 f5 f4 f3 f2 f1 f0
ef ee ed ec eb ea e9 e8 e7 e6 e5 e4 e3 e2 e1 e0
df de dd dc db da d9 d8 d7 d6 d5 d4 d3 d2 d1 d0
cf ce cd cc cb ca c9 c8 c7 c6 c5 c4 c3 c2 c1 c0
bf be bd bc bb ba b9 b8 b7 b6 b5 b4 b3 b2 b1 b0
af ae ad ac ab aa a9 a8 a7 a6 a5 a4 a3 a2 a1 a0
9f 9e 9d 9c 9b 9a 99 98 97 96 95 94 93 92 91 90
8f 8e 8d 8c 8b 8a 89 88 87 86 85 84 83 82 81 80
7f 7e 7d 7c 7b 7a 79 78 77 76 75 74 73 72 71 70
6f 6e 6d 6c 6b 6a 69 68 67 66 65 64 63 62 61 60
5f 5e 5d 5c 5b 5a 59 58 57 56 55 54 53 52 51 50
4f 4e 4d 4c 4b 4a 49 48 47 46 45 44 43 42 41 40
3f 3e 3d 3c 3b 3a 39 38 37 36 35 34 33 32 31 30
2f 2e 2d 2c 2b 2a 29 28 27 26 25 24 23 22 21 20
1f 1e 1d 1c 1b 1a 19 18 17 16 15 14 13 12 11 10
0f 0e 0d 0c 0b 0a 09 08 07 06 05 04 03 02 01 00
SPI slave says: read page 0 from the MOSI line:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f
40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f
50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f
60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f
70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f
80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f
90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f
a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af
b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf
c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf
d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df
e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef
f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff

If you look at the communication with a logic analyzer, you should see this:

spi master slave logic
Figure 2. Data capture as seen in Saleae Logic.

List of Files

CMakeLists.txt

CMake file to incorporate the example in to the examples build tree.

spi_master/spi_master.c

The example code for SPI master.

spi_slave/spi_slave.c

The example code for SPI slave.

Bill of Materials

Table 1. A list of materials required for the example

Item

Quantity

Details

Breadboard

1

generic part

Raspberry Pi Pico

2

https://www.raspberrypi.com/products/raspberry-pi-pico/

M/M Jumper wires

8

generic part