hd44780/ftdi: Fix a bug in ftdi_HD44780_backlight() and add backlight

switching capability for 4bit mode. Document the different BL line level
and a small switching circuit for it.

ChangeLog

@@ -23,6 +23,7 @@ v0.5dev (ongoing development)
  * picolcd: Use libusb-1.0 asynchronous transfers to fix missed keys (M. Jones)
  * sed1520: Make it work without using an external inverter
  * lis driver: Make the last pixel row work (add lastline option parsing)
+ * hd44780/ftdi: Fix error in setting backlight and add backlight for 4bit mode
 
 v0.5.5
  + sed1330 driver: Add support for HG25504 (L. Lagendijk)

docs/lcdproc-user/drivers/hd44780.docbook

@@ -1872,6 +1872,39 @@ The wiring of the control lines can optionally be reconfigured,
 please look at the driver source if you really need that.
 </para>
 
+<note><para>
+The backlight line is driven high when the backlight is <literal>on</literal>.
+Therefore the standard backlight circuit (<xref linkend="hd44780-connections-backlight.circuit"/>)
+will not work. Use the following instead.
+</para></note>
+
+<figure id="hd44780-ftdi-backlight">
+<title>hd44780/ftdi: Backlight Wiring</title>
+<screen>
+<![CDATA[
+                                             O 5V
+                                             |
+                                             +--------o 15 backlight
+
+                                             +--------o 16 GND backlight
+                                             |
+                           small resistor   .-.
+                           10 - 47 ohm      | |
+                           depending on     | |
+                           display          '-'
+                                             |
+                                             |c
+                          ___            b |/
+     BL pin o------------|___|-------------|
+                          4k7              |\
+                                       BC547 |e
+                                             |
+                                            === GND
+]]>
+</screen>
+</figure>
+
+
 <para>
 Alternatively you can use a single channel FTDI FT245BM USB &lt;-&gt; parallel
 FIFO chip and use the display in its 4 bit mode.
@@ -1948,6 +1981,13 @@ FIFO chip and use the display in its 4 bit mode.
       <entry>RW</entry>
       <entry>5</entry>
     </row>
+    <row>
+      <entry>D7</entry>
+      <entry>18</entry>
+      <entry></entry>
+      <entry>BL</entry>
+      <entry>Backlight (optional)</entry>
+    </row>
   </tbody>
 </tgroup>
 </table>
@@ -1967,6 +2007,7 @@ ftdi_mode=4
 ftdi_line_EN=0x10
 ftdi_line_RS=0x20
 ftdi_line_RW=0x40
+ftdi_line_backlight=0x80
 ]]>
 </screen>
 </example>

server/drivers/hd44780-ftdi.c

@@ -6,12 +6,13 @@
    wiring example:
    FTDI chip: D0 D1 D2 D3 D4 D5 D6 D7
               |  |  |  |  |  |  |  |
-   HD44780:   D4 D5 D6 D7 EN RS RW n/c
+   HD44780:   D4 D5 D6 D7 EN RS RW BL
 
    in LCDd.conf we then need to define
     ftdi_line_EN=0x10
     ftdi_line_RS=0x20
     ftdi_line_RW=0x40
+    ftdi_line_backlight=0x80
 
    RW of your display can either be connected to D6 or GND.
 \endverbatim
@@ -197,8 +198,9 @@ ftdi_HD44780_senddata(PrivateData *p, unsigned char displayID, unsigned char fla
 	unsigned char buf[4];
 	unsigned char portControl = 0;
 
+	portControl = 0x00 | p->backlight_bit;
 	if (flags == RS_DATA) {
-	    portControl = p->ftdi_line_RS;
+	    portControl |= p->ftdi_line_RS;
 	}
 
 	buf[0] = ((ch >> 4) & 0x0F) | portControl | p->ftdi_line_EN;
@@ -228,18 +230,28 @@ ftdi_HD44780_senddata(PrivateData *p, unsigned char displayID, unsigned char fla
 void
 ftdi_HD44780_backlight(PrivateData *p, unsigned char state)
 {
+    unsigned char buf[1];
+    int f;
+
+    p->backlight_bit = state ? p->ftdi_line_backlight : 0;
+    buf[0] = p->backlight_bit;
+
     if (p->ftdi_mode == 8) {
-	int f;
+	f = ftdi_write_data(&p->ftdic2, buf, 1);
-
-	p->backlight_bit = state ? p->ftdi_line_backlight : 0;
-
-	f = ftdi_write_data(&p->ftdic2, &state, 1);
 	if (f < 0) {
 	    p->hd44780_functions->drv_report(RPT_ERR, "failed to write: %d (%s). Exiting",
 				      f, ftdi_get_error_string(&p->ftdic2));
 	    exit(-1);
 	}
     }
+    else {
+	f = ftdi_write_data(&p->ftdic, buf, 1);
+	if (f < 0) {
+	    p->hd44780_functions->drv_report(RPT_ERR, "failed to write: %d (%s). Exiting",
+				  f, ftdi_get_error_string(&p->ftdic));
+	    exit(-1);
+	}
+    }
 }