PIC Microcontoller Memory Method

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:spameuroclass-Remove- 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