spisoft.rst

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.

Build variables

.. envvar:: SPISOFT_DELAY_VARIABLE

   default: 0 (disabled)

   This setting must be enabled in order to use :cpp:func:`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 :cpp:class:`SPISoft` constructor,
   or by calling :cpp:func:`SPISoft::setDelay`.

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

.. envvar:: 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

.. doxygengroup:: soft_spi
   :content-only:
   :members: