correct logic hazard in latch and load nybble; was transitioning control lines at the same time as enable, causing spurious behaviour on less-tolerant displays

improve timing parameters in init sequences for better display compatibility

correct (row,col) -> address calculation; affects 4-line displays

update comments in header and readme

drivers/lcd/pcf8574_lcd_backpack.c

@@ -63,9 +63,9 @@
 
 /* timing characteristics of the LCD interface */
 
-#define DELAY_US_NYBBLE0    200
+#define DELAY_US_NYBBLE0    20
-#define DELAY_US_NYBBLE1    100
+#define DELAY_US_NYBBLE1    10
-#define DELAY_US_WRITE      35
+#define DELAY_US_WRITE      40
 #define DELAY_US_HOMECLEAR  1500
 
 /* HD44780 commands */
@@ -259,7 +259,7 @@ static inline uint8_t rc2addr(FAR struct pcf8574_lcd_dev_s *priv,
        * of first line, and fourth line is a continuation of second.
        */
 
-      return (row - 2) * 0x40 + (col - priv->cfg.cols);
+      return (row - 2) * 0x40 + (col + priv->cfg.cols);
     }
 }
 
@@ -383,9 +383,14 @@ static void latch_nybble(FAR struct pcf8574_lcd_dev_s *priv, uint8_t nybble,
   en_bit = 1 << priv->cfg.en;
   rs_bit = rs ? (1 << priv->cfg.rs) : 0;
 
-  /* Put the nybble, preserving backlight, reset R/~W and set EN and maybe RS */
+  /* Put the nybble, preserving backlight, reset R/~W and maybe RS */
 
-  lcddata = prepare_nybble(priv, nybble) | priv->bl_bit | en_bit | rs_bit;
+  lcddata = prepare_nybble(priv, nybble) | priv->bl_bit | rs_bit;
+  pca8574_write(priv, lcddata);
+
+  /* Now set EN */
+
+  lcddata |= en_bit;
   pca8574_write(priv, lcddata);
   up_udelay(DELAY_US_NYBBLE0);  /* setup */
 
@@ -419,9 +424,14 @@ static uint8_t load_nybble(FAR struct pcf8574_lcd_dev_s *priv, bool rs)
   rs_bit = rs ? (1 << priv->cfg.rs) : 0;
   rw_bit = 1 << priv->cfg.rw;
 
-  /* Put highs on the data lines, preserve, set R/~W and set EN and maybe RS */
+  /* Put highs on the data lines, preserve, set R/~W and maybe RS */
 
-  lcddata = prepare_nybble(priv, 0x0f) | priv->bl_bit | en_bit | rw_bit | rs_bit;
+  lcddata = prepare_nybble(priv, 0x0f) | priv->bl_bit | rw_bit | rs_bit;
+  pca8574_write(priv, lcddata);
+
+  /* Now set EN */
+
+  lcddata |= en_bit;
   pca8574_write(priv, lcddata);
   up_udelay(DELAY_US_NYBBLE0);  /* setup */
 
@@ -538,7 +548,7 @@ static void lcd_init(FAR struct pcf8574_lcd_dev_s *priv)
 {
   /* Wait for more than 15 ms after Vcc for the LCD to stabilize */
 
-  usleep(20000);
+  usleep(50000);
 
   /* Perform the init sequence.  This sequence of commands is constructed so
    * that it will get the device into nybble mode irrespective of what state
@@ -548,7 +558,7 @@ static void lcd_init(FAR struct pcf8574_lcd_dev_s *priv)
    * the remainder of operations.
    */
 
-  /* Send Command 0x30, set 8-bit mode, and wait > 4.1 ms*/
+  /* Send Command 0x30, set 8-bit mode, and wait > 4.1 ms */
 
   latch_nybble(priv, 0x30>>4, false);
   usleep(5000);
@@ -556,15 +566,17 @@ static void lcd_init(FAR struct pcf8574_lcd_dev_s *priv)
   /* Send Command 0x30, set 8-bit mode, and wait > 100 us */
 
   latch_nybble(priv, 0x30>>4, false);
-  usleep(200);
+  usleep(5000);
 
   /* Send Command 0x30, set 8-bit mode */
 
   latch_nybble(priv, 0x30>>4, false);
+  usleep(200);
 
   /* now Function set: Set interface to be 4 bits long (only 1 cycle write for the first time). */
 
   latch_nybble(priv, 0x20>>4, false);
+  usleep(5000);
 
   /* Function set: DL=0;Interface is 4 bits, N=1 (2 Lines), F=0 (5x8 dots font) */
 

drivers/lcd/pcf8574_lcd_backpack_readme.txt

@@ -225,7 +225,7 @@ Example of programming a character image:
 Now character '\x04' will display as an 'up arrow'.
 
 Note, you might consider avoiding the use of code point 0x00 unless
-absolutely needed, because the embedded nul character can cause
+you absolutely need it, because the embedded nul character can cause
 problems.  The driver, and write() apis are binary, and unaffected,
 but things like printf() and puts() assume C-style strings, and are
 affected.
@@ -237,9 +237,16 @@ Troubleshooting
   bus timeouts that suggest a non-responsive slave.
 * Check your board wiring and configuration specification.  Buzz
   out the lines if you have to.
-* Con't forget to check the 'contrast' potentiometer.  The voltage
+* Remember to set the (ros,cols) geometry in pcf8574_lcd_backpack_config_s
-  at the central wiper should be approximately 0.29 V.  The useful
+  before registration of the driver, since this cannot be determined
-  range of voltages at this pin is very narrow, and outside that
+  programmatically.
-  range there will be nothing visible on the display, so most of the
+* If the driver registration step seems to 'hang' it could be the I2C
-  turn range of the pot is non-useful.  Much of human life has been
+  driver performing retries due to no response from the LCD backpack.  Check
-  wasted in the rediscovery of this farcically idiotic
+  the address.  Turning on debug output for I2C can help make this visible.
+* Don't forget to check the 'contrast' potentiometer.  The voltage at the
+  central wiper should be approximately 0.3 V - 2.4 V, but the actual value
+  is is dependent on the physics of the attached LCD module.  The useful
+  range of voltages at this pin for any given LCD is quite narrow, and
+  outside that range there will be nothing visible on the display, so most
+  of the turn range of the pot is non-useful.  It's less 'contrast' and
+  more 'LCD segment drive bias'.

include/nuttx/lcd/pcf8574_lcd_backpack.h

@@ -58,12 +58,15 @@
  * Pretty much anything on the market except 4x40, which really consists of two
  * separate 2x40 controllers, and the I2C backpack doesn't support those due
  * to the second 'E' line being needed.
+ * Additionally, you still need to set the (row,col) geometry explicitly, since
+ * there is not a means of determining this dynamically.
  * Consider these 'informative'.
- * XXX Note, actual testing has been done on LCD_I2C_BACKPACK_CFG_MJKDZ only,
+ * XXX Note, actual testing has been done on LCD_I2C_BACKPACK_CFG_MJKDZ
- * the others come from online research.
+ * and LCD_I2C_BACKPACK_CFG_SAINSMART only, the others come from online
+ * research.
  */
 
-/* board marked 'mjkdz' */
+/* board marked 'mjkdz' and Arduino-IIC-LCD GY-LCD-V1 */
 
 #define LCD_I2C_BACKPACK_CFG_MJKDZ {0x20,4,5,6,0,1,2,3,7,false,0,0}