nec 21a display module (skyper)

nec 21a display modules are controlled using an on-board controller sed1561.

picture of skyper (edition of nec 21a pager by german 'Telekom') source picture (author: Denis Apel) taken from Wikipedia / Skyper (Pager)

pinout for nec 21a display module 1 VDD +3.5V (Chip power supply) 2 (nc) not connected 3 (nc) not connected 4 VSS connect to GND 5 /RES low active Reset input 6 /CS low active Chip Select 7 A0 Command (LOW) or display data (HIGH) 8 ? unknown 9 ? unknown 10 BUT4 Button 4 11 BUT3 Button 3 12 BUT2 Button 2 13 BUT1 Button 1 14 D0 Data0 - Data7 15 D1 16 D2 17 D3 18 D4 19 D5 20 D6 21 D7 22 /RD Read enable input (active-low) 23 /WR Write enable input (active-low) 24 ? unknown 25 ? unknown annotations driving of power/message LED is more or less unknown driving of background light is unknown (pager i got had defective RAM and did never enlight background light - so i could not measure out how to drive it) as i couldn't acquire the socket for the display connector, i had to unsolder it from the mainboard of the pager

nec 21a display module showing a test image	output generated by multidisplay
output generated by graphlcd	another output generated by graphlcd

the wiring is based on an SED133x-wiring called 'PowerLCD'.
as i didn't know of any other projects like this one (connecting an NEC 21a display module to the parallel port) i had to chose a wiring-'standard' for myself.

signals:

 
                                  top view

                                                         ____ 
                                                        |    \
                                                   VDD  | =   |
                                                        |   = |  Sensor
                                                  (nc)  | =   |
                                                        |   = |  ?
                                                  (nc)  | =   |
                                                        |   = |  /WR
                                                   VSS  | =   |
                                                        |   = |  /RD
                                                  /RES  | =   |
                                                        |   = |  D7
                                                   /CS  | =   |
                                                        |   = |  D6
           --------------                           A0  | =   |
          |              |                              |   = |  D5
          |              |                           ?  | =   |
          |  LCD-        |                              |   = |  D4
          |  Controller  |                           ?  | =   |
          |              |                              |   = |  D3
          |  SED 1561    |                        BUT4  | =   |
          |              |                              |   = |  D2
          |              |                        BUT3  | =   |
          |              |                              |   = |  D1
          |              |                        BUT2  | =   |
          |              |                              |   = |  D0
          |              |                        BUT1  | =   |
          |              |                              |____/   
          |              |                              
           --------------       
                     
                     
   Remarks:                  
    display module is hard-wired to 8080-mode and parallel interface mode
                     
                                        
 
 
   LCD        Description
   ---------------------------------
   D0 - D7    Data0 - Data 7
   /WR        Write enable input (active-low)
   /RD        Read enable input (active-low)
   /RES       Reset (active-low)
   /CS        Chip select input (active-low)
   A0         Command (LOW) or display data (HIGH)
   VDD        3.5V (Chip power supply)
   VSS        0V

    

circuit:

part list:

    Part#   Type                Value                  Annotation
    ---------------------------------------------------------------------------------------
    R1      Resistor            eg. 270 Ohm  
    R2      Resistor            eg. 470 Ohm
    C1, C2  Capacitor           some uF                not mandatory, for smoothing input voltage
    IC1     Var. Voltage Reg.   LM317 (or similar)     Vout should result in 3.5 V

    R3-R14  Resistor            1 kOhm       

    R15     Resistor            10 kOhm
    C3      Capacitor           10 uF 

    

remarks:

R1 and R2 affect V_out of IC1 and are calculated using the following formula:

V_out = 1.25 * (1 + (R2 / R1))

usually (according to the data sheet of LM317) R1 should be 240 Ohm. i chose 270 Ohm.
together with R2 = 470 Ohm i get V_out = 3.425 V.

annotations

sed1561 supports a max. resolution of 132x65, but nec21a display modules only use 132 x 32.
the last line (#64) is used for displaying symbols (support and explanations will be added in a future version).

pages vs. rows

the display is organised in colums and pages: 8 pixel-rows are combined to a so called page.
as only whole bytes can be transferred to the display, a single pixel cannot be changed without knowing its 7 'neighbour' pixels.
so a display buffer was introduced in the library. pixel changes are all done in this buffer and in a separate step only changed 'page bytes' are transferred (using an optimising algorithm) to the display.

using the third parameter of serdisp_init(), driver specific options and custom wiring can be set.
common information about passing options and customisable wiring can be found in a separate document.

    examples:
    
      serdisp_init(sdcd, "NEC21A", "ROTATE=90;INVERT=YES");
    

  0 .. no delay
  1 .. one call of gettimeofday()
 >1 .. delay at least x nanoseconds