Table of Contents
SPI
Different variants exist. Usually SPI refers to the Motorola Serial Peripheral Interface. Other variants are the Texas Instruments Serial protocol (SSP) and the National Instruments Microwire.
Protocol
Most significant bit first
Modes
There are 4 Modes. Defied by the two settings “clock polarity” CPOL and “clock phase” CPHA.
“clock polarity” CPOL = 0:
- Clock line is low when idle.
“clock polarity” CPOL = 1:
- Clock line is high when idle.
“clock phase” CPHA = 0:
- first data bit is output on /CS activating (going low).
- data bits change when clock goes to idle level.
- data bits are read when clock comes from idle level.
“clock phase” CPHA = 1:
- first data bit starts on first edge after /CS activates.
- data bits change when clock comes from idle level.
- data bits are read when clock goes to idle level.
| mode | CPOL | CPHA | write bit | read bit |
|---|---|---|---|---|
| 0 | 0 | 0 | clock goes low, and when /CS activates | clock goes high |
| 1 | 0 | 1 | clock goes high | clock goes low |
| 2 | 1 | 0 | clock goes high, and when /CS activates | clock goes low |
| 3 | 1 | 1 | clock goes low | clock goes high |
SPI group
This group defines all used Serial Peripheral Interface (SPI) interfaces.
Each SPI interface is a sub group with the SPI name as the group name.
"SPI": {
"encoder_spi": {
"pad_sck": "PA5",
"pad_miso": "PA6",
"pad_mosi": "PA7",
"pad_ncs": "PA15",
"peripheral" : "SPI0",
"role": "master",
"communication_mode": "duplex",
"frame_format": "motorola",
"clock_polarity": "idle_low",
"clock_phase": "sample_on_leading_edge",
"bit_order": "msb_first",
"baud_rate": "10 MHz",
}
}
pad_sck
Serial Clock.
type: String
pad_mosi
“Master Out Slave In” data signal.
type: String
pad_miso
“Master In Slave Out” data signal.
type: String
pad_ncs
Chip Select signal (Low active).
type: String
peripheral
name of the SPI peripheral to use.
type: String
role
Is either master(host) or slave(device).
type: enum
default: master
communication_mode
Describes the communication. Possible values are:
- duplex : the traditional mode using MISO and MOSI to transmit and receive at the same time.
- half-duplex : meaning the same wire between master and slave is used for both directions.
- receive-only : send line is not used
- send-only : receive Line is not used.
type: enum
default: duplex
frame_format
Describes the communication. Possible values are:
- motorola : the “normal” SPI.
- ti :
type: enum
default: motorola
clock_polarity
Also often called CPOL. Describes the communication. Possible values are:
- idle_low
- idle_high
type: enum
default: idle_low
clock_phase
Also often called CPHA. Describes the communication. Possible values are:
- sample_on_leading_edge : samples the data on the edge from idle value to non-idle value.
- sample_on_trailinging_edge : samples the data on the edge from non-idle value to idle value.
type: enum
default: sample_on_leading_edge
SPI Mode
Sometimes the documentation talks about a SPI Mode with the value of 0 to 3. This mode maps to the phase and polarity as described in the following table:
| Mode | CPOL | CPHA | description |
|---|---|---|---|
| 0 | 0 | 0 | clock: idle_low, phase: sample_on_leading_edge |
| 1 | 0 | 1 | clock: idle_low, phase: sample_on_trailing_edge |
| 2 | 1 | 0 | clock: idle_high, phase: sample_on_leading_edge |
| 3 | 1 | 1 | clock: idle_high, phase: sample_on_trailing_edge |
bit_order
Describes the communication. Possible values are:
- msb_first : most significant bit first (bits on the line : 76543210 )
- lsb_first : least significant bit first (bits on the line: 01234567 )
type: enum
default: msb_first
baud_rate
Is the number of bits per second exchanged on the data lines. Expressed as frequency of the clock line.
type: int (Hz)
API
The part “signal_name” in the described functions will be replaced by the name of the signal. E.g. if your signal is called “audio” then the API function “signal_name_transfer” will become “audio_transfer”.
void signal_name_transfer(const uint8_t *data_send,
const uint8_t *data_receive,
const uint32_t length )
Will send “length” bytes from the buffer “data_send” out on MOSI and will receive the same amount of bytes from MISO into the buffer “data_receive”.