The 16C74A to 256KB SIMM interface (can easily adapt up to 4 MB) ================================================================ As a primary source of information I used the Motorola Memory Device Data Q3/91. All comments and explanations are brief, email me if you come to any dificulties. mailto:@spam@euroclassTakeThisOuT at spampha.pvtnet.cz In my setup the SIMM draws about 10 mA of supply current. The setup uses only 8 bits of the SIMM, but the 9 bit versions can be used with just 2 resistors added (but utilizes still only 8). It can be expanded to accomodate also 1M and 4M x 8 or 9 bit SIMMs. My version uses portB, portD, RE0, RC0, RC1, RC2 and one 8-bit latch - 74HC573. From other PIC peripherals the TMR0 is used for refresh purposes. The PIC runs with 10 MHz crystal, but this is because of other circuits, the refresh routine uses only some 7% of the time with safe margin, so lower crystal frequencies can be used as well. The above calculation is the worst case - based on 8 ms refresh time. There are also 256KB SIMMs with 64 ms refresh and than the interrupt can be servised less often or the PIC run at even lower frequencies. The 74HC573 has its inputs connected to the port D, the latch enable (or clock) input to RE0, the output enable is grounded. The outputs of latches are named I0-I7 in the following text. Here are the connections to the SIMM module: SIMM PIC or HC573 pin# name 1 VCC power +5 V 2 CAS RC0 3 DQ0 RB0 4 A0 I0 5 A1 I1 6 DQ1 RB1 7 A2 I2 8 A3 I3 9 VSS power 0 V 10 DQ2 RB2 11 A4 I4 12 A5 I5 13 DQ3 RB3 14 A6 I6 15 A7 I7 16 DQ4 RB4 17 A8 RD0 18 A9* RD1 19 A10** RD2 20 DQ5 RB5 21 W RC2 22 VSS power 0 V 23 DQ6 RB6 24 no connection 25 DQ7 RB7 26 Q8*** no connection 27 RAS RC1 28 CAS8*** resistor 10 K to +5V 29 D8*** resistor 10 K to +5V 30 VCC power +5 V * on 1M SIMM only ** on 4M SIMM only *** on x9 SIMM only Registers: FSR: used by SIMMWR Variables: RASCNT: 1 byte, used during interrupt, cannot be shared TEMP: 1 byte, used by SIMMWR SIMM0: the upper address byte SIMM1: the middle address byte SIMM2: the lower address byte SIMM0-2 address range: 0 to 03ffff for 256K SIMM 0 to 0fffff for 1M SIMM 0 to 3fffff for 4M SIMM Pin definitions #define _CAS PORTC,RC0 #define _RAS PORTC,RC1 #define _W PORTC,RC2 #define _LE PORTE,RE0 Peripherals initialization: portB as input, pullups on portD as output RC0,1,2 as outputs, all at HI level RE0 as output, at LOW level Timer0 prescaler 32, interrupt enabled (for 10 MHz clock results in < 4 ms interrupt period) Before first communication after power up the RAS signal should be cycled 8 times to initialize the dynamic nodes in DRAMS. As an alternative, one could try to invoke the refresh by setting the T0IF. TMR0 interrupt service: INT:save STATUS and W, select RP0 INTT0: BCF _T0IF ;TIMER 0 INTERRUPT MOVLW D'32' ;Performes the CAS before RAS refresh, 256 cycles every 4 ms MOVWF RASCNT I2: BCF _CAS ;of course this can be shortened BCF _RAS ;and initial RASCNT value increased, BSF _RAS ;but I still have plenty of EPROM left BCF _RAS BSF _RAS BCF _RAS BSF _RAS BCF _RAS BSF _RAS BCF _RAS BSF _RAS BCF _RAS BSF _RAS BCF _RAS BSF _RAS BCF _RAS BSF _RAS BSF _CAS DECFSZ RASCNT,F GOTO I2 restore STATUS and W RETFIE ;WRITE SIMM ;ADDRESS IN SIMM0-2 ;DATA IN W ;assumes RP0 is selected SIMMWR: MOVWF TEMP ;store data in temporary location BCF _T0IE ;disable TMR0 interrupt, so that the MOVLW TRISB ;refresh does not interfere with write MOVWF FSR ;make FSR to point to TRISB, avoid bank switching MOVF SIMM2,W ;latch lowest address byte in HC573 MOVWF PORTD BSF _LE BCF _LE SW0: MOVF SIMM0,W ;isolate bit 16 of the address ANDLW 0x01 MOVWF PORTD ;output it on port D BCF _RAS ;activate RAS MOVF SIMM1,W ;latch middle address byte in HC573 MOVWF PORTD BSF _LE BCF _LE SW1: RRF SIMM0,W ;isolate bit 17 of the address ANDLW 0x01 MOVWF PORTD ;output it on port D BCF _W ;activate W CLRF INDF ;make PORTB output MOVF TEMP,W ;output data on PORTB MOVWF PORTB BCF _CAS ;cycle CAS - performs write operation BSF _CAS COMF INDF,F ;make PORTB input BSF _W ;deactivate W BSF _RAS ;deactivate RAS BSF _T0IE ;enable refresh RETURN DW 0x3FFF ;READ SIMM ;ADDRESS IN SIMM0-2 ;OUTPUT DATA IN W ;assumes RP0 is selected SIMMRD: BCF _T0IE ;disable TMR0 interrupt, so that the ;refresh does not interfere with read MOVF SIMM2,W ;latch lowest address byte in HC573 MOVWF PORTD BSF _LE BCF _LE SR0: MOVF SIMM0,W ;isolate bit 16 of the address ANDLW 0x01 MOVWF PORTD BCF _RAS ;activate RAS MOVF SIMM1,W ;latch middle address byte in HC573 MOVWF PORTD BSF _LE BCF _LE SR1: RRF SIMM0,W ;isolate bit 17 of the address ANDLW 0x01 MOVWF PORTD BCF _CAS ;activate CAS GOTO $+1 ;wait for the data to be valid MOVF PORTB,W ;read data BSF _RAS ;deactivate RAS BSF _CAS ;deactivate CAS BSF _T0IE ;enable refresh RETURN DW 0x3FFF The modification for 1M or 4M SIMM involves change in treatment of SIMM0. The code should be changed as follows (not tested): 1M SIMM, SW0 and SR0: MOVF SIMM0,W ANDLW 0x03 MOVWF PORTD 1M SIMM, SW1 and SR1: RRF SIMM0,W ANDLW 0x06 MOVWF PORTD RRF PORTD,F 4M SIMM, SW0 and SR0: MOVF SIMM0,W ANDLW 0x07 MOVWF PORTD 4M SIMM, SW1 and SR1: RLF SIMM0,W ANDLW 0x70 MOVWF PORTD SWAPF PORTD,F
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