MAX7219 behaves strangely with various PICs and config.


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  1. #1
    Join Date
    Feb 2013
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    Default MAX7219 behaves strangely with various PICs and config.

    Hello.

    Here's sample code I'm using:

    Code:
    ' Program: MAX_CNT.BAS
    ' *************************************************************
    ' * For use with EXPERIMENTING WITH THE PICBASIC PRO COMPILER *
    ' *							      *
    ' *  This source code may be freely used within your own      *
    ' *  programs. However, if it is used for profitable reasons, *
    ' *        please give credit where credit is due.	      *
    ' *  And make a reference to myself or Rosetta Technologies   *
    ' *							      *
    ' *			Les. Johnson			      *
    ' *************************************************************
    '
    ' This Program demonstrates the use of the MAX7219, seven segment decoder driver
    ' It also incorporates placing of Decimal point.
    ' ***********************************************************************
    
    	Include "Modedefs.bas"
    
    ' ** Set Xtal Value in mHz **
    
    	Define	OSC	4		' Set Xtal Frequency
    
    ' ** Declare Pins Used **
    	Clk     Var	PortB.0		' Data is clocked on rising edge of this pin
    	Dta  	Var	PortB.1		' Bits are shifted out of this pin
    	Load    Var	PortB.2		' Transfers data to LEDs when Pulsed
    
    ' ** Declare Constants **
    	Decode_Reg 	Con	9	' Decode register, a 1 turns on BCD decoding for each digit. 
    	Lum_Reg		Con	10	' Intensity register.
    	Scan_Reg	Con	11	' Scan-limit register.
    	Switch_Reg	Con	12	' On/Off Register.
    	Test_Reg	Con	15	' Test mode register (all digits on, 100% bright)
    
    '	Max_Digit	Con	5	' Amount of LED Displays being used.
    
    ' ** Declare Variables **
      	Counter		Var	Word	' Variable used for the Demo Counting routine
    	Max_Disp	Var	Word	' 16-bit value to be displayed by the MAX7219
    	Max_Dp		Var	Byte	' Digit number to place Decimal point (0-4)
    	Register	Var	Byte	' Pointer to the Internal Registers of the MAX7219
    	R_Val		Var	Byte	' Data placed in Each Register
    	Digit		Var	Byte	' Position of individual numbers within MAX_Disp (0-3)
    	Position	Var	Byte	' Position of each LED display (1-4)
    
    ' ** INITIALIZE THE MAX7219 ** 
    ' Each register address is sent along with its setting data. 
    ' Because the MAX7219 expects to see a packet of 16 bits, then the LOAD pin is pulsed
    ' Set the scan limit to 3 (4 digits, numbered 0-3)
    ' Set the Brightness to 5
    ' BCD decoding to the lower 4 digits 
    ' Switch the display on. 
    ' Turn Off test mode
    
    	Register=Scan_Reg		' Point to the Scan Register
    	R_Val=3				' send 3, (Four LED Displays 0-3)
    	Gosub Transfer			' Transfer this 16-bit Word to the MAX7219
    
    	Register=Lum_Reg		' Point to the Luminance Register
    	R_Val=5				' Send 5, (Value for Brightness)
    	Gosub Transfer			' Transfer this 16-bit Word to the MAX7219
    
    	Register=Decode_Reg		' Point to BCD Decode Register
    	R_Val=%00011111			' Decode the first 5 digits
    	Gosub Transfer			' Transfer this 16-bit Word to the MAX7219
    
    	Register=Switch_Reg		' Point to the Switch Register 
    	R_Val=1				' Set to One, (switches the display ON)
    	Gosub Transfer			' Transfer this 16-bit Word to the MAX7219
    
    	Register=Test_Reg		' Point to the Test Register
    	R_Val=0				' Reset to Zero, (turns off Test mode)
    	Gosub Transfer			' Transfer this 16-bit Word to the MAX7219
    
    
    ' 		***** MAIN PROGRAM *****
    ' This loop increments and then decrements a 16-bit number 
    ' And displays it on Four LED Displays
    ' The value to be displayed is held in the variable "Max_Disp"
    ' The Position of the decimal point is held in Max_DP (0-4)
    
    	Max_Dp=3			' Display number for Decimal Point
    	
    Again:	For Counter=1 to 9999		' Increment Counter
    	Max_Disp=Counter		' Load Max_Disp with Value of Counter
    	Gosub Display			' Display the Value of Counter
    	Pause 150			' Delay, so we can see whats happening
    	Next				' Close the Loop
    
    	For Counter=9999 to 1 step -1	' Decrement Counter
    	Max_Disp=Counter		' Load Max_Disp with Value of Counter
    	Gosub Display			' Display the Value of Counter
    	Pause 150			' Delay, so we can see whats happening
    	Next				' Close the Loop		
    	Goto Again			' Do it Indefinately
    
    
    ' ** Subroutines **
    ' Display the Value held in the Variable "MAX_DISP" on the four LED's
    ' The value held in "MAX_DP" places the decimal point on that LED (0-3)
    ' Sending the value 15 blanks the display, this allows Zero suppression
    ' By setting bit-7 of the value sent to the individual LED displays, the decimal point
    ' Is illuminated
    
    Display:
    	Digit=0						' Start at Digit 0 of Max_Disp Variable
    	For Position=4 to 1 step -1			' Start at Farthest Right of Display 		
    	Register=Position				' Place Position into Register
    	R_Val=Max_Disp Dig Digit			' Extract the individual numbers from Max_Disp			
    	If Max_Disp<10 and Position=3 then R_Val=15	' Zero Suppression for the second digit
    	If Max_Disp<100 and Position=2 then R_Val=15	' Zero Suppression for the Third digit
    	If Max_Disp<1000 and Position=1 then R_Val=15	' Zero Suppression for the Forth digit
    	If Max_Disp<10000 and Position=0 then R_Val=15	' Zero Suppression for the Fifth digit
    	If Digit=Max_Dp then R_Val.7=1			' Place the decimal point, held in Max_DP
    	Gosub Transfer					' Transfer the 16-bit Word to the MAX7219
    	If Digit>=3 then Digit=0			' We only need the first four digits
    	Digit=Digit+1					' Point to next Digit within Max_Disp
    	Next Position					' Close the Loop
    	Return						' Exit from subroutine
    
    ' Send a 16-bit word to the MAX7219 
    Transfer:
      	Shiftout Dta,Clk,msbfirst,[Register,R_Val] ' Shift Out the Register first, then the data
    	High Load				' The data is now acted upon
    @	Nop
    @	Nop					' A small delay to ensure correct clocking times
    	Low Load				' Disable the MAX7219 
    	Return					' Exit from Subroutine
    I have several MAX7219, hooked to different displays, and with different PICs, results are different:

    12F683 - no display @ 4mhz, works OK for 3-4 minutes at 8mhz
    16F628A - counts to 5 or 8 (depends on MAX chip), then goes off
    16F870 - counts fine, however, 2nd digit (from the left) sometimes displays 3, instead of 0. With another MAX chip, screen goes off when displaying 28, and may come back on 33.

    Soldering is correct, MAX7219 -s are genuine, obtained from digikey.

  2. #2
    Join Date
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    Default Re: MAX7219 behaves strangely with various PICs and config.

    Found the reason, but don't know why it happens.

    It depends on Max_dp variable.
    If it equals 5 or 4, then there's no problem. If 3, 2, or 1, it causes problem. if DP=3, then hangs on 2800 (around), if DP=2, then hangs on 280, if DP=1, then hangs on 8.

    What might be the reason?

  3. #3
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    Default Re: MAX7219 behaves strangely with various PICs and config.

    If anyone interested, the reason for this issue was noisy power supply. Added 100uf capacitor at 7219 power pins - problem gone.

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