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:-Remove-euroclassSPAM 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
See also:
| file: /Techref/microchip/picsimm.htm, 7KB, , updated: 2001/2/8 10:39, local time: 2024/11/14 09:22,
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Ashley Roll has put together a really nice little unit here. Leave off the MAX232 and keep these handy for the few times you need true RS232! |
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