pcd8544

pcd8544-based displays can be found in some monochrome nokia-cellphones. with some technical skills, an independant, small display with background light can be gained out of these phones at hardly any costs.

   serdisp_init(sdcd, "PCD8544", "");
   serdisp_init(sdcd, "LPH7366", "");
    

at least two different types of displays and circuit boards exist (i called them "type 1" and "type 2", maybe there are even more).

(digits in brackets show the marks in the following pictures) type 1: the display is connected to the circuit board [2] using metallic pins [1] leds are soldered on the other side of the circuit board [3] they enlight the display through holes in the circuit board [3] the display has 8 pins pin #1 is rightmost (rear view of display, connectors at the bottom line)   tiny cables could be soldered directly onto the metallic pins recommentation: design a circuit board for leds and display connector on your own because the circuit board can hardly be recycled   update: some displays use a different method to connect the display module to the circuit board: contact pads and a separate "transmitter". it should be possible to solder wires directly onto the contact pads or to use an elastomer type 2: the display is connected to the circuit board [2] using an elastomer ('lcd-connector' or 'rubber pad with conducting joints') [1] leds are soldered on the same side of the circuit board [3] underneath the position of the display on the other side of the circuit board there are no electronic parts the display itself is "clipped" [4] onto the circuit board the display has 9 pins (additional possibility to connect an external oscillator) pin #1 is rightmost (rear view of display, elastomer at the bottom line)   the circuit board of the cellphone can be 'recycled' by simply cutting out the interesting part (see picture below)

different display types in bigger view + pin 1 positions

ad type 2: recycling the circuit board using a 'dremel' (or similar). red lines indicate where to cut through

ad type 2: i used a flex cable because i damaged the connector pad. as all leds are wired together, you only have to find two positions where to solder the wires so that the leds can be connected. i drilled two tiny holes through the circuit board (red arrows) and soldered a wire directly to a cathode and the other wire to an anode

type 2 display finished and working	output generated by graphlcd

the initial wiring is based on the wiring of an optrex 323 to the parallel port. as i didn't know of any other projects like this (with pcd8544-displays connected to the parallel port) i had to chose a wiring-'standard' for myself.

2004-04-29

a severe mistake is now corrected: the pin-order was swapped in the photos (and at two positions in the description)!
when you look at the display in rear view with connectors at the bottom-line, pin 1 is rightmost (not leftmost)!

but: ascii arts in the source code of the library (serdisp_specific_pcd8544.h) did not contain this mistake!

2004-11-28

when reviewing the source code i noticed that signal 'CS' is never used and may be safely connected to GND. i updated circuits and descriptions to reflect this (CS slipped in from optrex323 when i adapted it's driver for pcd8544).

note: the functionality of serdisplib is NOT affected through this update (because CS wasn't used anyway)

2005-02-12

some pcd8544-based displays (especially newer ones) run unstable if Vout is not connected. inserting a capacitor avoids this. thanks to Michael Bülte for explaining this problem to me

added circuits using hardware reset (R/C-circuit) instead of /RES-wire

signals:

    Type 1 displays

                     +-------------------------+
                     |     1 2 3 4 5 6 7 8     |
                     |     # # # # # # # #     |
                     |  ===#=#=#=#=#=#=#=#===  |             1 .. VDD
                     +--=====================--+             2 .. SCLK
                     |                         |             3 .. SI
                     |                         |             4 .. D/C
                     |        rear view        |             5 .. /CS
                     |  connector is visible   |             6 .. GND
                     |                         |             7 .. Vout
                     |         LPH7779         |             8 .. /RES
                     |                         |
                     +-------------------------+


    Signal name  PC            LCD     Signal name
    ---------------------------------------------------------
    Data 0       2              3      SI   serial data input of LCD
    Data 1       3              2      SCLK serial clock line of LCD
    Data 2       4              4      D/C  (or sometimes named A0) command/data switch
    Data 5       7              8      /RES active low reset
    Data 7       9              -           Backlight (optional, not on display)
    Ground      18              6      GND  Ground for printer port and VDD
                --              1      VDD  +V (? mA) Chip power supply
                --              5      /CS   active low chip select (connected to GND)
                --              7      Vout output of display-internal dc/dc converter


    Type 2 displays

 
                     +--------------------------+
                     |     1 2 3 4 5 6 7 8 9    |
                     |     # # # # # # # # #    |             1 .. VDD
                     |  ===#=#=#=#=#=#=#=#=#==  |             2 .. SCLK
                     +--======================--+             3 .. SI
                     |                          |             4 .. D/C
                     |                          |             5 .. /CS
                     |        rear view         |             6 .. Osc
                     |   connector is visible   |             7 .. GND
                     |                          |             8 .. Vout
                     |         LPH7366          |             9 .. /RES
                     |                          |
                     +--------------------------+
 
 
    Signal name  PC            LCD     Signal name
    ---------------------------------------------------------
    Data 0       2              3      SI   serial data input of LCD
    Data 1       3              2      SCLK serial clock line of LCD
    Data 2       4              4      D/C  (or sometimes labelled A0) command/data switch
    Data 5       7              9      /RES active low reset
    Data 7       9              -           Backlight (optional, not on display)
    Ground      18              7      GND  Ground for printer port and VDD
                --              1      VDD  +V (up to 7.4 mA) Chip power supply
                --              5      /CS  active low chip select (connected to GND)
                --              6      Osc  external clock (connected to VDD)
                --              8      Vout output of display-internal dc/dc converter


    

circuits:

part list:

    Part#   Type                Value                  Annotation
    ---------------------------------------------------------------------------------------
    R1      Resistor            eg. 270 Ohm  
    R2      Resistor            eg. 330 Ohm
    C1, C2  Capacitor           some uF                not mandatory, for smoothing input voltage
    IC1     Var. Voltage Reg.   LM317 (or similar)     Vout should result in > 2.7V and < 3.3 V

    R3-R7   Resistor            1 kOhm       
    R8      Var.Resistor        eg. 100 Ohm            or higher value if needed;
                                                       or simply a resistor with fixed value
    C3      Capacitor           100 nF                 not mandatory; needed, if display is unstable
                                                       (eg: only every 2nd row is visible, ...) 
    T1      Transistor          eg. BC547    
    D1      Diode               eg. 1N4148   

    

remarks:

type 2 displays (lph 7366) have 9 pins, type 1 displays (lph 7677, 7779) only have 8 pins. both circuits are illustrated above.

R7, R8, T1, D1 are needed in combination with LED background light only!

C1 and C2 are not mandatory, but at least C1 is recommended for smooth power supply.

C3 is only needed if the display runs unstable (possible erraneous effect: only every 2nd row is shown).   thanks to Michael Bülte for this hint

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 did not have such a resistor so i chose 270 Ohm. together with R2 = 330 Ohm i get V_out = 2.7777777 V.
that is at the lower limit of the specification of pcd8544. so maybe you should chose better values for R1 and R2.

i also tested a PJ 1084 (smd lowdrop adj. voltage reg.) as a replacement for the LM317 and it worked fine.
attention: different order of pins!

hardware reset vs. software reset:

to save one wire it is possible to replace the /RES-wire through a R/C-circuit.

part-list is the same as above with the following exceptions:

    Part#   Type                Value                  Annotation
    ---------------------------------------------------------------------------------------
    R3-R6   Resistor             1 kOhm       
    R7      Resistor            10 kOhm
    C4      Capacitor           10 uF 

     

pro

one wire less

contra

display can no longer be reset by software

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, "PCD8544", "WIRING=3;INVERT=YES");
      serdisp_init(sdcd, "LPH7690", "WIRING=Rifer");
    

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