Continue to Site

MLRC question

Status
Not open for further replies.

wejos

Member
what happens if i will leave out the MLRC part of the circuit (dont include the 20k resistor and the 472 ceramic), you think it will still function?

and i have noticed also that some projects use 1k resistor between the (say) RB0 and LED, some projects do not esp on the RC ones. should i also?

Last edited:

blueroomelectronics

Well-Known Member
For what schematic?

ericgibbs

Well-Known Member
what happens if i will leave out the MLRC part of the circuit (dont include the 20k resistor and the 472 ceramic), you think it will still function?

and i have noticed also that some projects use 1k resistor between the (say) RB0 and LED, some projects do not esp on the RC ones. should i also?

hi,
It depends upon how you have assigned the MCLR pin in the CONFIG of your program.

Whats the PIC type.?

wejos

Member
PIC is 16f84A. here's the schematic and im using it for serial communication.

Last edited:

wejos

Member
heres the code and ive simulatide this and worked.

Code:
LIST	p=16F84		;tell assembler what chip we are using
include "P16F84.inc"		;include the defaults for the chip
ERRORLEVEL	0,	-302	;suppress bank selection messages
__CONFIG   _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC ;sets the configuration settings (oscillator type etc.) or RC_OSC if thats what your using

Rcv_Byte     equ    0x0C
Bit_cntr     equ    0x1C
Delay_Count  equ    0x0D
STATUS	     equ	0x03
PORTA	     equ	0x05
PORTB	     equ	0x06
TRISA	     equ	0x85
TRISB	     equ	0x86

org	0x000

Initialise	CLRF	PORTA
CLRF	PORTB
BSF     STATUS,5
MOVLW   b'00000000'
MOVWF   PORTB
BCF     STATUS,5
CALL	SER_INIT
Loop		CALL	Rcv_RS232
MOVWF	PORTB
goto	Loop

SER_INIT
BSF     STATUS,5
MOVLW   b'00000001'
MOVWF   PORTA
BCF     STATUS,5
RETURN

Rcv_RS232   BTFSC   PORTA, 0x00
GOTO    Rcv_RS232
CALL    Start_Delay
BTFSC   PORTA,0x00
GOTO    Rcv_RS232
MOVLW   0x08
MOVWF   Bit_cntr
CLRF    Rcv_Byte
Next_RcvBit CALL    Bit_Delay
BTFSS   PORTA,0x00
BCF     STATUS,C
BTFSC   PORTA,0x00
BSF     STATUS,C
RRF     Rcv_Byte,f
DECFSZ  Bit_cntr,f
GOTO    Next_RcvBit
CALL    Bit_Delay
MOVF    Rcv_Byte,W
RETURN

Start_Delay MOVLW   0x0C
MOVWF   Delay_Count
Start_Wait  NOP
DECFSZ  Delay_Count,f
GOTO    Start_Wait
RETURN

Bit_Delay   MOVLW   0x18
MOVWF   Delay_Count
Bit_Wait    NOP
DECFSZ  Delay_Count,f
GOTO    Bit_Wait
RETURN

end

wejos

Member
taken from nigel's tutorial i just changed some gprs etc etc...

ericgibbs

Well-Known Member
taken from nigel's tutorial i just changed some gprs etc etc...

Hi,
As Nigel's circuit shows it, the MCLR res/cap is required.

Attachments

• AAAimage02.gif
52.1 KB · Views: 232

blueroomelectronics

Well-Known Member
The 10pf caps on the crystal may be a little low. 22pf is a better choice. Also you'll need 0.1uf caps near the VSS & VDD on the PIC
I've avoided the cap on MCLR as it can mess up ICD programming.

wejos

Member
thanks for your speedy reply guys. will it damage the PIC provided ive left out this part?

smanches

New Member
Probably not, but it will probably randomly reset on you. The resistor MUST be there for this to not happen. Reset pins should never be left floating.

Why do you want to leave it out?

Last edited:

wejos

Member
The 10pf caps on the crystal may be a little low. 22pf is a better choice. Also you'll need 0.1uf caps near the VSS & VDD on the PIC
I've avoided the cap on MCLR as it can mess up ICD programming.

k i will. ty. i have 33 pf?

wejos

Member
Probably not, but it will probably randomly reset on you. The resistor MUST be there for this to not happen. Reset pins should never be left floating.

Why do you want to leave it out?

k fair enough. so thats the purpose of this resistor and cap.

Nigel Goodwin

Super Moderator
k fair enough. so thats the purpose of this resistor and cap.

Generally with PIC's you can simply connect it to Vdd - you MUST NOT leave it floating.

thank you guys

wejos

Member
i bought the .1 uf 50v caps for the decoupling but it's electrolytic. should i follow the polarity when i mounted it? i have a spare .47 uf 50v caps can i use this also to decouple the rs232? im kind of short of caps. thank you.

ericgibbs

Well-Known Member
i bought the .1 uf 50v caps for the decoupling but it's electrolytic. should i follow the polarity when i mounted it? i have a spare .47 uf 50v caps can i use this also to decouple the rs232? im kind of short of caps. thank you.

The 100nF will be OK, neg term to 0V

470nF for the RS232 are a bit small, but they should work OK.

wejos

Member
ty eric. i shared the decoupling cap to both the PIC and RS232 and still worked.

I'll try use the 470nf for the MAX also.

wow! 9600 is fast.

thank you guys for everything.

millwood

Banned
neither the cap nor the resistor is needed: they form a low pass filter to improve resistance to random interference, and the cap holds the mlrc pin low briefly during power on.

Status
Not open for further replies.

Replies
4
Views
1K
Replies
3
Views
994
Replies
9
Views
3K
Replies
6
Views
948
Replies
1
Views
865