reverse SS HIGH/LOW in Scanner_ShiftRegsPISOMultiRow::scan()

doc/keybrd_library_user_guide.md

@@ -142,6 +142,7 @@ The following is a listing of items to check when a new keybrd sketch or keyboar
 
 Development-environment items to check:
 * If the keyboard has an I/O expander, power cycle (replug the USB) after loading the HEX file.
+* Sometimes sketch will not loading properly if two instances of Arduino IDE are open.
 * For compile error:
 ```
     'KEY_A' was not declared in this scope

src/Scanner_ShiftRegsPISOMultiRow.cpp

@@ -21,32 +21,44 @@ Initializes shift register's shift/load pin.
 */
 void Scanner_ShiftRegsPISOMultiRow::begin()
 {
+    digitalWrite(slaveSelect, HIGH);            //initialize ??only needed for first scan
     SPI.begin(); //todo move this to constructor or init()
 }
 
 /* scan() strobes the row's strobePin and returns state of the shift register's input pins.
 strobePin is Arduino pin number connected to this row.
 Bit patterns are 1 bit per key.
+
+Scanner_ShiftRegsPISOMultiRow class was tested on two sets of 74HC165 shift registers
+and 74AHC1G126 tri-state buffer chips
+
+74HC165 is not an SPI device.
+The 74HC165 SH/LD polarity is the inverse of SPI slave-select convention.
+Most SPI chips will high-Z their MISO pin when their slave select signal is high.
+To use SPI-like protocol, 74HC*126 will high-Z the MISO pin when the slave select signal is low.
+
+SPI.beginTransaction() and SPI.endTransaction() are not needed,
+but would be needed if trackball uses interrupts.
 */
 read_pins_t Scanner_ShiftRegsPISOMultiRow::scan(const uint8_t strobePin)
 {
     read_pins_t readState = 0;                  //bits, 1 means key is pressed, 0 means released
 
-    SPI.beginTransaction( SPISettings(5000000, MSBFIRST, SPI_MODE0) ); //control SPI bus, 5 MHz
-
-    digitalWrite(slaveSelect, LOW);             //load parallel inputs to registers
-
     digitalWrite(strobePin, strobeOn);          //strobe on
 
-        delayMicroseconds(3);                       //time to stabilize photo-transistor todo need?
+    //SPI.beginTransaction( SPISettings(5000000, MSBFIRST, SPI_MODE0) ); //control SPI bus, 5 MHz
-        delayMicroseconds(50); //todo for sr2_LEDs_strobe.ino
+
+    delayMicroseconds(20);                      //photo-transistor at 3.3v needs 20 ms to stablize
+
     digitalWrite(slaveSelect, HIGH);            //shift the data toward a serial output
 
     digitalWrite(strobePin, strobeOff);         //strobe off
 
     SPI.transfer(&readState, byte_count);
 
-    SPI.endTransaction();
+    //SPI.endTransaction();
+
+    digitalWrite(slaveSelect, LOW);             //load parallel inputs to registers
 
     return readState;
 }

src/Scanner_ShiftRegsPISOSingleRow.cpp

@@ -36,9 +36,9 @@ read_pins_t Scanner_ShiftRegsPISOSingleRow::scan(const uint8_t strobePin)
     read_pins_t readState = 0;                  //bits, 1 means key is pressed, 0 means released
 
     //read all the column pins
-    digitalWrite(slaveSelect, LOW);             //load parallel inputs to the register
     digitalWrite(slaveSelect, HIGH);            //shift the data toward a serial output
     SPI.transfer(&readState, byte_count);
+    digitalWrite(slaveSelect, LOW);             //load parallel inputs to the register
 
     return readState;
 }