51 MCU common knowledge points

First of all,

the timer/
counter is very important. It is the heart of the microcontroller.
First of all, let's talk about some concepts.
Clock frequency: fosc (provided by the crystal oscillator, 51 is generally 12Mhz and 11.0592Mhz)
12 division: fosc/12 (I set it to fT here)
Machine cycle: T=1/fT

There are two configurations for the timer.
The first is the working mode: timer/counter.
The second is the four working modes.

Then there are two registers
TMOD&TCON
working mode register and control register

working mode register TMOD
(1) GATE - When the gate bit
GATE=0, only the control bit TRX (X=0, 1) starts the timer/counter.
When GATE=1, the high level on TRX (X=0, 1) and the interrupt pin (INT0 and INT1) jointly starts the timer/counter.
(2) M0, M1 -
There are four encodings for the working mode selection bits M0 and M1, corresponding to the four working modes. Corresponding relationship
M1 M0 Working mode
0 0 Mode 0, 13-bit timer/counter
0 1 Mode 1, 16-bit timer/counter
1 0 Mode 2, 8-bit timer/counter 1 with 8-bit initial value automatically reloaded
1 Mode 3, only applicable to T0, divided into two 8-bit counters, T1 stops counting
(3) C/T——Counter mode and timer mode selection bit
C/T=0, timer mode
C/T=1, counter mode, the counter counts the external pulse (negative jump) of the external input pin T0 (P3.4 pin) or T1 (P3.5 pin).

Timer/counter control register TCON
The byte address of TCON is 88H, which can be bit-addressed, and the bit address is 88H~8FH.
The lower four bits are related to interrupts, and the upper four bits
(1) TF0, TF1——T0, T1 count overflow flag bit
When the counter counts overflow, this position is "1". When using the query mode, this bit is used as a status bit for the CPU to query, but it should be noted that after the query is valid, the bit should be cleared to 0 in a timely manner by software. When using the interrupt mode, this bit is used as an interrupt request flag bit and is automatically cleared to 0 by hardware after entering the interrupt service program .
(2) TR0, TR1 - counting operation control bit
TR1 bit (or TR0 bit) = 1, a necessary condition for starting the timer/counter, and is also related to the GATE bit.
TR1 bit (or TR0 bit) = 0, turns off the timer/counter.

Regarding the initial value calculation of several working modes,
the first working mode is a 13-bit timer.
Its maximum value is 213=81922
13
=8192.
The initial value calculation is,
for example, to time 1ms.
1ms is the time, which corresponds to the number of steps taken by the microcontroller.
First, calculate how much time one step takes, which is the machine cycle 12/fosc.
You can calculate a total of fosc/(12*1000) steps.
Subtract this number of steps from the maximum value of the timer to get the initial value of the timer.
The second working mode is similar.
The third is automatic reload. The values ​​of its high and low registers are the same.