Baur rate of AT89S52

maniac84 said:

I'm trying to use the 9600 baud rate with a 20Mhz xtal. The value given in the table shows I must use F5h and the SMOD=1. But there's SMOD0 and SMOD1 registers. What is the value for SMOD0 and SMOD1 if SMOD=1?

Click to expand...

maniac84 said:

Why is it is SM0 and SM1 instead of SMOD0 and SMOD1?

Click to expand...

IanP said:

Notation depends on teacher/book/school ..
SM0=SMOD0
SM1=SMOD1
for lazier people, less letters ...

:wink:
IanP

Click to expand...

maniac84 said:

Noted.
I want to use it in 9600 baud rate on timer 1. So, I put SCON = 80h and TH1=F5h. But it won't work.
When I try on 4800 baud rate which is SCON=00h and TH1=F5h. It work!
Do you guys know what's wrong?

Click to expand...

MOV A, PCON                                ;A P CON
SET B ACC.7                                 ;make D7 = 1
MOV PCON, A                                 ;SMOD = 1, double baud rate
MOV TMOD, #20 H                        ;Timer 1 mode 2, auto reload
MOV THI,#0F5 H                                        
MOV SCON, #50 H

IanP said:

There is a confusion here; let’s explain it ---
SMOD is bit 7 of PCON register and when set to “0” {SMOD=0} divides 1/12 of the clock frequency by 32.
When set to “1” {SMOD=1} divides 1/12 of the clock frequency by 16.
This is still within the PCON register.

SM0 and SM1 are bits of the SCON register.
Don’t confuse SMOD with SMs …

As far as the setting for 9600bps goes, serial mode1 is set by SM0=o and SM1=1, that makes the SCON register set to #50h (with the REN bit also set to “1”), not #80h.
The TMOD register should be set to #20h …
And bit7 of PCON (SMOD) set to “1” …
9600= 2 x 20MHz /[32 x12 x (256-TH1)]
TH1 =10.8 --- closest 11--- 0xf5
Something like:

Code:

MOV A, PCON                                ;A P CON
SET B ACC.7                                 ;make D7 = 1
MOV PCON, A                                 ;SMOD = 1, double baud rate
MOV TMOD, #20 H                        ;Timer 1 mode 2, auto reload
MOV THI,#0F5 H                                        
MOV SCON, #50 H

Now it should work …

:wink:
IanP

Click to expand...

MOV TMOD, #20 H                        
MOV THI,#0F5 H                                        
MOV SCON, #50 H

MOV A, PCON                              
SET B ACC.7                               
MOV PCON, A
MOV TMOD, #20 H                        
MOV THI,#0F5 H                                        
MOV SCON, #50 H