Implement sequential read/write for 16-bit operations

[olkal]

Hi, this PR is implementing sequential read/write for 16-bit operations as described in the data sheet 3.2.1 page 13.
See test data below from running the MCP23S17_performance.ino sketch on an ESP32-S3@240mhz.

HW_SPI@8mhz before:
	TEST write16(mask): 13.50
	TEST read16(): 13.50

HW_SPI@8mhz with sequential read/write:
	TEST write16(mask): 10.50
	TEST read16(): 10.50

SW_SPI before:
	TEST write16(mask): 29.50
	TEST read16(): 29.50

SW_SPI with sequential read/write:
	TEST write16(mask): 20.00
	TEST read16(): 20.00

[RobTillaart]

Great addition!
Will look into it later this week

[RobTillaart]

This addition was on my todo list, so I'm happy you implemented and tested it.

The POR (power on reset) value of the SEQOP bit is zero ==> sequential read/write mode.

However as we will not be sure if the value is set to zero, I propose to clear that bit explicitly

Add in the begin() function. line 73++ something like

uint8_t reg = readReg(MCP23017_IOCR);
if (reg & MCP23017_IOCR_SEQOP)  //  check if already zero
{
  reg &= ~MCP23017_IOCR_SEQOP;  //  clear SEQOP bit for sequential read/write
  if (! writeReg(MCP23017_IOCR, reg)) return false;
}

[olkal]

Yes, I agree. Should I also remove line 73 then? // if (! writeReg(MCP23017_IOCR, MCP23017_IOCR_SEQOP)) return false; I don't see any any reason to keep it.

[RobTillaart]

Please remove line 73 as it has no function

[RobTillaart]

LGTM
The 16 bit calls now only transfer 4 bytes instead of 6 bytes which can be seen in your performance measurements.
Will squash and merge it now and bump the version number tomorrow.

[RobTillaart]

@olkal
Did performance test on AVR UNO with the optimized 16 bit and the relative gain is again ~30%.
Especially for SW SPI the absolute improvement is substantial.

function	0.5.2	0.5.3	notes
SW SPI write16	450	300
SW SPI read16	446	298
HW SPI write16	42	26	1 MHz
HW SPI read16	42	28	1 MHz
HW SPI write16	28	18	2 MHz
HW SPI read16	30	18	2 MHz
HW SPI write16	22	16	4 MHz
HW SPI read16	24	16	4 MHz
HW SPI write16	20	14	8 MHz
HW SPI read16	20	14	8 MHz