Software SPI

Software SPI is likely to be of use only for ESP8266, but is enabled for all architectures.

The ESP8266 can manage a minimum of about 70ns between bit edges, with a maximum of about 1.8 MBit/s at normal CPU clock frequency or 2.5 MHz if CPU clock set to fast.

Configuration Variables

SPISOFT_DELAY_VARIABLE

default: 0 (disabled)

This setting must be enabled in order to use SPISoft::setDelay(). The base clock speed (i.e. delay=0) is reduced to about 1.4 MBit/s (2.1 MBit/s for fast CPU).

The appropriate delay factor can be provided in the SPISoft constructor, or by calling SPISoft::setDelay().

With the ESP8266, then the appropriate delay factor is calculated automatically from the speed passed in SPISettings. This will override any previously set delay factor. Use a speed of 0 to use the manually configured delay value.

SPISOFT_DELAY_FIXED

default: 0 (disabled) maximum: 10

Adds the requested number of ‘NOP’ CPU instructions to every clock transition.

Has less impact than enabling variable delays.

Use variable delays if 10 is insufficient.

Will be ignored if variable delays are enabled.

Performance

The following information has been obtained by measurement. The test application was built using:

make SPISOFT_CALIBRATE=1 SPISOFT_DELAY_VARIABLE=1 CPU_FAST=0

then flashed to an ESP-12F. An oscilliscope was hooked up to the SCK output and the maximum frequency at each delay setting noted. (Not too awkward, just note down the frequency each time the figure changes.)

Entering these into the spisoft.ods spreadsheet allows the values to be charted and co-efficients calculated.

Curiously values from 8 upward form a straight line so we can calculate those. Smaller values are non-linear so a small lookup table is used.

Repeat for CPU_FAST=1.

Note that these figures are concerned with maximum (i.e. burst) frequencies at each delay setting, since that is what devices are sensitive to.

80 MHz CPU

.	Frequency (kHz)
Delay	Variable	Fixed
0	1357	2288
1	1315	1869
2	1215	1824
3	1129	1780
4	1055	1744
5	990	1709
6	932	1639
7	881	1575
8	819	1511
9	743	1462
10	680	1408
11	626	1338
12	578	1294
20	367
30	251
40	192
50	155
60	129
70	112
80	98
90	87
100	79

160 MHz CPU

.	Frequency (kHz)
Delay	Variable	Fixed
0	2133	2857
1	2126	2510
2	2106	2500
3	1958	2439
4	1861	2429
5	1745	2372
6	1668	2353
7	1573	2294
8	1482	2283
9	1357	2242
10	1250	2222
11	1160	2174
12	1081	2165
20	702
30	488
40	374
50	303
60	255
70	220
80	193
90	172
100	156

API Documentation

class SPISoft : public SPIBase 

#include <SPISoft.h>

Software-based SPI master.

Intended for ESP8266 due to limited I/O but will work on any architecture.

Constructors

SPISoft(): Default constructor uses same pins as hardware SPI.

inline SPISoft(uint8_t miso, uint8_t mosi, uint8_t sck, uint8_t delay = 0)

Specify pins to use plus optional delay.

Delay is ignored if code is not compiled with SPISOFT_DELAY < 0.

inline SPISoft(const SpiPins &pins, uint8_t delay = 0): Specify pins plus optional delay.

SPISoft(uint8_t delay): Use default pins but provide a delay.

Public Functions

virtual bool begin() override: Initialize the SPI bus by setting SCK and MOSI to outputs, pulling SCK and MOSI low.

inline virtual void end() override: Disable the SPI bus (leaving pin modes unchanged).

virtual void endTransaction() override: Stop using the SPI bus. Normally this is called after de-asserting the chip select, to allow other libraries to use the SPI bus.

virtual uint32_t transfer32(uint32_t val, uint8_t bits = 32) override

Send/receive a word of variable size.

Word is transferred either MSB first (bits-1) or LSB first (bit 0) depending on the currently applied bitOrder setting.

Parameters:

val – Word to send
bits – Size of word

virtual void transfer(uint8_t *buffer, size_t size) override

Send/receive a variable-length block of data.

Parameters:

buffer – IN: The data to send; OUT: The received data
size – Number of bytes to transfer

inline void setDelay(uint8_t delay)

Set delay factor for the SCK signal. Impacts SPI speed.

Requires code to be compiled with SPISOFT_DELAY < 0.

ESP8266 only: The delay will be automatically calculated for a requested clock speed when begin() or beginTransaction() are called. To use only the manually programmed delay, set the clock speed to zero.

inline virtual bool loopback(bool enable) override

For testing, tie MISO <-> MOSI internally.

Note: Not included in std Arduino lib

inline uint8_t transfer(uint8_t val)

Send/receive one byte of data.

Parameters:: val – The byte to send
Return values:: uint8_t – The received byte

void transfer(uint8_t *buffer, size_t size) = 0

Send/receive a variable-length block of data.

Parameters:

buffer – IN: The data to send; OUT: The received data
size – Number of bytes to transfer