please dont rip this site

TB011

Using SRAM With A PIC16CXXX

Author: Rick Evans

Microchip Technology Inc.

INTRODUCTION

There are applications where a significant amount of data memory is required beyond what is in the micro-controller. For example, buffering communications data, creating large volatile tables and arrays. One interesting application is voice storage and playback. Some applications require relatively low frequencies, so a fast address/data bus and expensive FSRAM are not necessary.

This application note uses the PIC16C74. Since the PIC16C74 does not have an external address/data bus, one was created using the I/O ports. A software implementation of a multiplexed address/data bus is more than adequate for some applications. This application note discusses the read and write speeds

achievable with the PIC16C74 running at 20 MHz, hardware connections necessary, and software routines for reading and writing to external memory.

IMPLEMENTATION

The multiplexed address/data bus was created using nineteen port pins (Figure 1). PORTD is address lines 0 through 7 multiplexed with data lines 0 through 7. PORTB is the upper address lines 8-15. PORTE<0> is the RD, PORTE<1> is WR and PORTE<2> is ALE (address latch enable). One SRAM chip is address range 0000h-7FFFh, and the other SRAM chip is address range 8000h-FFFFh. The chip selecting is done using A15 and A15. A 74LS373 latch is needed to demultiplex the low order address/data bus.

The software needed to initialize the ports is in the subroutine init_muxbus (Appendix A). The initial states of the address lines and bus control signals are shown in the comments of the subroutine.

Ó  1997 Microchip Technology Inc.

DS91011A-page 1


TB011

READ CYCLE

The software to do the read cycle is very straight forward. The read cycle drives the address onto the 16-bit bus, then latches the low order address lines 0-7 on to the memory device (Figure 2). When ALE goes low, address 0-7 is latched. Then the low order bus lines 0-7 are changed to input pins. Next, the read pin (RD) goes low which turns on the output buffers of the memory device. Some time later, the data from the memory is driven on to the data lines 0-7. Then RD goes high and the output buffers of the memory device are disabled. Next, the low order data lines are changed back to outputs. The subroutine read_extmem shows the code used to emulate a read cycle on PORTB and PORTD.

You can ascertain from Table 1, that a slow SRAM can be used. There are three critical SRAM read cycle specifications:

TACC: Address access time TOE: Output enable time TDF: Data float time

The address access time TACC corresponds to the TAVDV (1.6 ms) of the emulated muxed bus. A FUJITSU MB84256C-70 has an access time of 70 ns. The output enable time TOE from the Fujitsu data sheet is 35 ns. The emulated muxed bus specification TRLDV (200 ns) corresponds to the TOE of the Fujitsu SRAM. The data float time TDF is only important if you are doing back to back bus cycles.

WRITE CYCLE

The write cycle moves data to the external SRAM. The address is driven on the sixteen address lines, then the low order address is latched by making ALE go high and then low. The data is moved to the low order address lines (data lines). Finally, the WR pin is driven low and then high. When write goes high, the data is written to the SRAM. Figure 3 shows the bus timing diagram for the write cycle.

The write cycle is even simpler to implement in software than the read cycle. The subroutine write_extmem shows code used to write out data to an external SRAM.

The write pulse width is an important specification in memory design as well. For the PIC16C74 emulated address/data bus the write pulse width is TWLWH. The value for the write pulse width is very large compared to actual address/data bus implementations. The TAVWH specification corresponds to the TAW specifica-tion of most memory devices. The emulated address/ data bus has relatively slow bus timings compared with most memory access times such as EPROMs and SRAMs.

SUMMARY

A 40-pin PIC16CXXX device such as the PIC16C74 can interface to external memory. This application note used sixteen address lines multiplexed with eight data lines to read/write to two external SRAMs. The read/ write cycle times can be calculated easily given the single cycle instruction architectures of the PIC16CXXX. Flash, EPROM and other parallel bus memory devices can be similarly utilized.

   TABLE 1: READ AND WRITE CYCLE BUS TIMINGS        


 
 
         Timing Description Minimum   Maximum  


 
 
      TCY Instruction cycle time @ 20 MHz 200 ns   DC  


 
 
      TLLLH ALE pulse width 1 TCY    


 
 
      TAVDV Address valid to data valid   7 TCY  


 
 
      TRLDV Read low to data valid 1 TCY    


 
 
      TRHDZ Read high to data float 0   1 TCY  


 
 
      TWLWH WRITE pulse width 1 TCY    


 
 
      TWHDX WRITE high to data no longer valid (data hold time)   2 TCY  


 
 
      TAVWH Address valid to write high   5 TCY  


 
 

DS91011A-page 2

Ó  1997 Microchip Technology Inc.


TB011

Ó  1997 Microchip Technology Inc.

DS91011A-page 3


TB011

APPENDIX A:PROGRAM LISTING: EXTERNAL MEMORY SUBROUTINES

;*******************************************************************

;* Initialize the Multiplexed Address/Data Bus ;*

;*

;*

;*******************************************************************

Init_MUXBUS

bsf STATUS,RP0 ;switch to bank 1 registers
clrf TRISB ;set A8-A15 as output
clrf TRISD ;set AD0-AD7 as output
movlw 0xf8  
andwf TRISE,F ;ALE,RD#,WR# as output
     
bcf STATUS,RP0 ;switch to bank 0 registers
     
clrf ADHIGH ;set A8-A15 to 0 (PORTB)
clrf ADLOW ;set AD0-AD7 to 0 (PORTD)
movlw 3 ;ale=0,rd#=1,wr#=1

     movwf PORTE return

;*******************************************************************

;* Read External Memory of muxed bus ;*

;*    
;********************************************************************
read_extmem    
   movf ADLOW,W ;save low order address
   movwf ADLOW_IMAGE  
     
   bsf PORTE,ALE ;ALE high for 200ns, RD#, WR# low
   bcf PORTE,ALE ;ALE goes low (A0-7 latched)
   bsf STATUS,RP0  
   movlw 0xff  
   movwf TRISD ;make PORTD input
   bcf STATUS,RP0  
     
   bcf PORTE,RD ;drop READ low
     
   movf ADLOW,W ;move read data from AD bus to w reg
   bsf PORTE,RD ;pull READ high (RD pulse is 400ns)
   bsf STATUS,RP0  
   clrf TRISD ;make PORTD (ADLOW) output again
   bcf STATUS,RP0  
     
   movwf w_image ;save READ data
   movf ADLOW_IMAGE,W ;restore low order address

     
     
     
     
DS91011A-page 4    

Ó  1997 Microchip Technology Inc.


TB011

movwf ADLOW ;on port
movf w_image,W ;restore READ data to w
     
return    

;*******************************************************************

;* Write to External Memory on muxed bus

;*******************************************************************
write_extmem    
   movwf w_image ;save w (data to write)
   movf ADLOW,W  
   movwf ADLOW_IMAGE ;save the low order address
   movf w_image,W ;restore w (data to write)
     
   bsf PORTE,ALE ;ALE high for 200ns, RD#,WR# low
   bcf PORTE,ALE ;latch lower address
   movwf ADLOW ;move write data to AD0-7
   bcf PORTE,WR ;WR# low for 200ns
   bsf PORTE,WR ;latch data in external memory
     
   movf ADLOW_IMAGE,W  
   movwf ADLOW ;restore low order address
     
   return    

     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
Ó 1997 Microchip Technology Inc.  

DS91011A-page 5


WORLDWIDE SALES & SERVICE

AMERICAS ASIA/PACIFIC
Corporate Office Hong Kong
Microchip Technology Inc. Microchip Asia Pacific
2355 West Chandler Blvd. RM 3801B, Tower Two
Chandler, AZ 85224-6199 Metroplaza
Tel: 602-786-7200 Fax: 602-786-7277 223 Hing Fong Road
Technical Support: 602 786-7627 Kwai Fong, N.T., Hong Kong
Web: http://www.microchip.com Tel: 852-2-401-1200 Fax: 852-2-401-3431
Atlanta India
Microchip Technology Inc. Microchip Technology India
500 Sugar Mill Road, Suite 200B No. 6, Legacy, Convent Road
Atlanta, GA 30350 Bangalore 560 025, India
Tel: 770-640-0034 Fax: 770-640-0307 Tel: 91-80-229-0061 Fax: 91-80-229-0062
Boston Korea
Microchip Technology Inc. Microchip Technology Korea
5 Mount Royal Avenue 168-1, Youngbo Bldg. 3 Floor
Marlborough, MA 01752 Samsung-Dong, Kangnam-Ku
Tel: 508-480-9990 Fax: 508-480-8575 Seoul, Korea
Chicago Tel: 82-2-554-7200 Fax: 82-2-558-5934
Microchip Technology Inc. Shanghai
333 Pierce Road, Suite 180 Microchip Technology
Itasca, IL 60143 RM 406 Shanghai Golden Bridge Bldg.
Tel: 630-285-0071 Fax: 630-285-0075 2077 Yan’an Road West, Hongiao District
Dallas Shanghai, PRC 200335
  Tel: 86-21-6275-5700
Microchip Technology Inc.  
  Fax: 86 21-6275-5060
14651 Dallas Parkway, Suite 816  
Dallas, TX 75240-8809 Singapore
Tel: 972-991-7177 Fax: 972-991-8588 Microchip Technology Taiwan
Dayton Singapore Branch
  200 Middle Road
Microchip Technology Inc.  
  #10-03 Prime Centre
Two Prestige Place, Suite 150  
  Singapore 188980
Miamisburg, OH 45342  
  Tel: 65-334-8870 Fax: 65-334-8850
Tel: 937-291-1654 Fax: 937-291-9175  
  Taiwan, R.O.C
Los Angeles  

EUROPE

United Kingdom

Arizona Microchip Technology Ltd. Unit 6, The Courtyard Meadow Bank, Furlong Road Bourne End, Buckinghamshire SL8 5AJ Tel: 44-1628-851077 Fax: 44-1628-850259

France

Arizona Microchip Technology SARL Zone Industrielle de la Bonde 2 Rue du Buisson aux Fraises 91300 Massy, France

Tel: 33-1-69-53-63-20 Fax: 33-1-69-30-90-79

Germany

Arizona Microchip Technology GmbH Gustav-Heinemann-Ring 125 D-81739 Müchen, Germany

Tel: 49-89-627-144 0 Fax: 49-89-627-144-44

Italy

Arizona Microchip Technology SRL Centro Direzionale Colleone Palazzo Taurus 1 V. Le Colleoni 1 20041 Agrate Brianza Milan, Italy Tel: 39-39-6899939 Fax: 39-39-6899883

JAPAN

Microchip Technology Intl. Inc. Benex S-1 6F

3-18-20, Shin Yokohama Kohoku-Ku, Yokohama Kanagawa 222 Japan

Tel: 81-4-5471- 6166 Fax: 81-4-5471-6122

5/8/97

Microchip Technology Inc. 18201 Von Karman, Suite 1090 Irvine, CA 92612 Tel: 714-263-1888 Fax: 714-263-1338

New York

Microchip Technology Inc. 150 Motor Parkway, Suite 416 Hauppauge, NY 11788

Tel: 516-273-5305 Fax: 516-273-5335

San Jose

Microchip Technology Inc.

2107 North First Street, Suite 590 San Jose, CA 95131 Tel: 408-436-7950 Fax: 408-436-7955

Toronto

Microchip Technology Inc. 5925 Airport Road, Suite 200

Mississauga, Ontario L4V 1W1, Canada Tel: 905-405-6279 Fax: 905-405-6253

Microchip Technology Taiwan 10F-1C 207 Tung Hua North Road Taipei, Taiwan, ROC

Tel: 886 2-717-7175 Fax: 886-2-545-0139

M

All rights reserved. © 1997, Microchip Technology Incorporated, USA. 6/97

Information contained in this publication regarding device applications and the like is intended for suggestion only and may be superseded by updates. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. The Microchip logo and name are registered trademarks of Microchip Technology Inc. in the U.S.A. and other countries. All rights reserved. All other trademarks mentioned herein are the property of their respective companies.

91011A-page 6 ã 1997 Microchip Technology Inc.


See also:


file: /Techref/microchip/an/91011a/result.htm, 43KB, , updated: 2002/6/27 17:48, local time: 2018/5/24 14:37,
TOP NEW HELP FIND: 
54.80.82.9:LOG IN

 ©2018 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?
Please DO link to this page! Digg it! / MAKE! / 

<A HREF="http://techref.massmind.org/techref/microchip/an/91011a/result.htm"> result</A>

After you find an appropriate page, you are invited to your to this massmind site! (posts will be visible only to you before review) Just type in the box and press the Post button. (HTML welcomed, but not the <A tag: Instead, use the link box to link to another page. A tutorial is available Members can login to post directly, become page editors, and be credited for their posts.


Link? Put it here: 
if you want a response, please enter your email address: 
Attn spammers: All posts are reviewed before being made visible to anyone other than the poster.
Did you find what you needed?

 

Welcome to massmind.org!

 

Welcome to techref.massmind.org!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  .