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Add note on dev board usage of SPI bus 0 (#1591)

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Mike Pye 2016-11-17 19:10:48 +00:00 committed by Marcel Stör
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docs/en/modules/spi.md
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All transactions for sending and receiving are most-significant-bit first and least-significant last.
For technical details of the underlying hardware refer to [metalphreak's ESP8266 HSPI articles](http://d.av.id.au/blog/tag/hspi/).
## A note on the SPI busses
The ESP hardware provides two SPI busses, with IDs 0, and 1, which map to pins
generally labelled SPI and HSPI.
If you are using any kind of development board which provides flash, then bus
ID 0 (SPI) is almost certainly used for communicating with the flash chip.
You probably want to choose bus ID 1 (HSPI) for your communication, as you
will have uncontended use of it.
## High Level Functions
The high level functions provide a send & receive API for half- and
full-duplex mode. Sent and received data items are restricted to 1 - 32 bit