+ Reply to Thread
Results 1 to 4 of 4
  1. #1

    Default 16f877A V 18F4550

    ok, after killing my last 877a i orderd a new 877a and a 4550...
    Acording to microchip, the 877a and the 4550 are not pin compatable...

    However, i disagre...
    with the following exceptions...

    Pin 18:
    877a: RC3 (SCK/SCL)
    4550: VUSB
    i havn't actually used pin 18 yet...

    Pin 14:
    877a: OSC2 (CLKO)
    4550: OSC2 (CLKO/RA6)
    oooh, if i use internal clock i get an extra IO pin...

    Pin 1:
    877a: MCLR (VPP)
    4550: MCLR (VPP/RE3)
    Question... how the hell can MCLR be used as an IO pin? as soon as it goes low (if input or output) your CPU resets...

    I'm aware their is a gneration gap between them...
    I'm aware their are internal changes...
    I'm aware their are more analog lines
    and I'm aware that the 4550 can run upto 48MHz (thats the main reason i baught it!)

    Anyone know of any other pin diferences that i've missed?


  2. #2
    Join Date
    Jul 2003
    Colorado Springs


    Quote Originally Posted by comwarrior View Post
    and I'm aware that the 4550 can run upto 48MHz (thats the main reason i baught it!)
    48Mhz isn't always such a good thing unless you'll be using USB.

    With almost any other 18F device you can run at 40Mhz.
    That extra 8Mhz doesn't really add much speed, but it really messes up the math when trying to do things.

    Instead of the nice round 0.1uS instruction cycle, it's now 0.083333... repeating.
    Then integer truncation causes small errors in just about everything you try to calculate against time.

    If you will be using USB, then it's just something you have to live with, unless you run the CPU at a different speed while still giving 48Mhz to the USB.

    Plus one bad thing about the 18F4550 is that you can't use both the USART and the MSSP module because the the RX and SDO share the same pin. SDO was moved from RC5 to make room for the USB D+ pin.

    If there's no USB ... the 18F4420/4520 is a great alternative to the 877A.
    Completely pin compatible, with lots of extras.

  3. #3


    thanks for that darrel...

    Actually, i currently have no plans to use the USB nor the USARTs but i may do at some point...

    i'm actually only using two ADC's in my curent project, however, i do have plans to use them all so the extra 5 will come in handy...
    The anoying thing is the extra ADC's are on a port i'm using for an LCD.. looks like it'll have to be moved...


  4. #4
    Join Date
    Oct 2009

    Default pin differences

    I've found many. It would be tedious to list all the pin differences. Basically, don't try to drop this chip into an 877 circuit unless you want to redesign the layout. I'm a beginner, and I found that out the hard way. Bleh.

    Here's what got me most:

    Port C pins 4 and 5 are digital input ONLY, and on reset they are normally disabled. Unless, that is, the USB module and USB tranciever are disabled.
    Last edited by MyBuddy; - 21st October 2009 at 19:36. Reason: add more info

Similar Threads

  1. 16F877A to 18F4550 comms
    By comwarrior in forum General
    Replies: 5
    Last Post: - 4th October 2009, 01:22
  2. 18F4550 to 18F2550
    By mpardinho in forum mel PIC BASIC Pro
    Replies: 2
    Last Post: - 28th September 2007, 23:13
  3. How can I turn off USB support in 18F4550
    By BrianT in forum mel PIC BASIC Pro
    Replies: 1
    Last Post: - 6th September 2007, 03:56
  4. Clock connection to DS1802 from 16F877A
    By coyotegd in forum mel PIC BASIC Pro
    Replies: 7
    Last Post: - 3rd January 2006, 18:52
  5. 1st 16F877A program in PICBasic
    By Borisw37 in forum mel PIC BASIC Pro
    Replies: 4
    Last Post: - 19th January 2005, 04:46

Members who have read this thread : 0

You do not have permission to view the list of names.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts