BUG: The SPI library does not handle HW CS for other bit lengths than 8.

[tclarsson]

BUG: The SPI library does not handle HW SC for other bit lengths than 8.
For example: There is a bug in the routine "transfer16".

Hardware: WEMOS ESP8266

If one uses transfer16 for 16 bit SPI transfer using HW CS, the SS/CS pin goes high after each 8 bit transfer. the 16bit word is therefore transferred as 2 x 8 bit bytes.

For the below to work:
SPI.setHwCs(true); //Use (automatic) HW SS/CS (GPIO 15 , D8)
SPI.setBitOrder(MSBFIRST);
SPI.transfer16(0x1234);

I had to write my own alternative "_transfer16" routine:

uint16_t _bs(uint16_t data) {
return(((data&0xFF)<<8)|((data>>8)&0xFF));
}

void _setDataBits(uint16_t bits) {
const uint32_t mask = ~((SPIMMOSI << SPILMOSI) | (SPIMMISO << SPILMISO));
bits--;
SPI1U1 = ((SPI1U1 & mask) | ((bits << SPILMOSI) | (bits << SPILMISO)));
}

uint16_t _transfer16(uint16_t data) {
if(!(SPI1C & (SPICWBO | SPICRBO))) data=_bs(data);
while(SPI1CMD & SPIBUSY) {}
_setDataBits(16);
SPI1W0 = data;
SPI1CMD |= SPIBUSY;
while(SPI1CMD & SPIBUSY) {}
data=SPI1W0;
if(!(SPI1C & (SPICWBO | SPICRBO))) data=_bs(data);
return (data);
}

[devyte]

@tclarsson is this issue still valid in latest git?

[UlliBien]

The bug is still there (Arduino/ESP8266, version 2.5.2). write16() does it correctly. I am not familiar with github and unfortunately can not make the change myself.

Change code to:
uint16_t SPIClass::transfer16(uint16_t data) {
while (SPI1CMD & SPIBUSY) {}
// Set to 16Bits transfer
setDataBits(16);
// Change byte order for MSBFIRST
if (!(SPI1C & (SPICWBO | SPICRBO))){
// MSBFIRST Byte first
SPI1W0 = (data >> 8) | (data << 8);
}
else {
// LSBFIRST Byte first
SPI1W0 = data;
}
// Start the transmission
SPI1CMD |= SPIBUSY;
// Wait for transmission to complete
while (SPI1CMD & SPIBUSY) {}
data = SPI1W0;
// Change byte order for MSBFIRST
if (!(SPI1C & (SPICWBO | SPICRBO)))
return (data >> 8) | (data << 8);
else
return data;
}

[thangktran]

@UlliBien I'll be working on this issue