Analog Devices ADuC812 manual Smod, Clk, RCAP2H RCAP2L

ADuC812

Timer 1 Generated Baud Rates

When Timer 1 is used as the baud rate generator, the baud rates in Modes 1 and 3 are determined by the Timer 1 overflow rate and the value of SMOD as follows:

Modes 1 and 3 Baud Rate =

(2SMOD/32) ⋅ (Timer 1 Overflow Rate)

The Timer 1 interrupt should be disabled in this application. The Timer itself can be configured for either timer or counter opera- tion, and in any of its three running modes. In the most typical application, it is configured for timer operation, in the Autoreload mode (high nibble of TMOD = 0010 Binary). In that case, the baud rate is given by the formula:

Modes 1 and 3 Baud Rate = (2SMOD/32) ⋅ (Core Clock/(12 ⋅ [256-TH1]))

Table XXI shows some commonly-used baud rates and how they might be calculated from a core clock frequency of 11.0592 MHz and 12 MHz. Generally speaking, a 5% error is tolerable using asynchronous (start/stop) communications.

Table XXI. Commonly-Used Baud Rates, Timer 1

Ideal	Core	SMOD	TH1-Reload	Actual	%
Baud	CLK	Value	Value	Baud	Error
9600	12	1	–7 (F9h)	8929	7
19200	11.0592	1	–3 (FDh)	19200	0
9600	11.0592	0	–3 (FDh)	9600	0
2400	11.0592	0	–12(F4h)	2400	0

Timer 2 Generated Baud Rates

Baud rates can also be generated using Timer 2. Using Timer 2 is similar to using Timer 1 in that the timer must overflow 16 times before a bit is transmitted/received. Because Timer 2 has a 16-bit Autoreload mode a wider range of baud rates is possible using Timer 2.

Modes 1 and 3 Baud Rate = (1/16) ⋅ (Timer 2 Overflow Rate)

Therefore, when Timer 2 is used to generate baud rates, the timer increments every two clock cycles and not every core machine cycle as before. Hence, it increments six times faster than Timer 1, and therefore baud rates six times faster are possible. Because Timer 2 has 16-bit autoreload capability, very low baud rates are still possible.

Timer 2 is selected as the baud rate generator by setting the TCLK and/or RCLK in T2CON. The baud rates for transmit and receive can be simultaneously different. Setting RCLK and/or TCLK puts Timer 2 into its baud rate generator mode as shown in Figure 34.

In this case, the baud rate is given by the formula:

Modes 1 and 3 Baud Rate -

(Core Clk)/(32 ⋅ [65536 – (RCAP2H, RCAP2L)])

Table XXII shows some commonly used baud rates and how they might be calculated from a core clock frequency of 11.0592 MHz and 12 MHz.

Table XXII. Commonly Used Baud Rates, Timer 2

Ideal	Core	RCAP2H	RCAP2L		Actual	%
Baud	CLK	Value	Value		Baud	Error
19200	12	–1 (FFh)	–20	(ECh)	19661	2.4
9600	12	–1 (FFh)	–41	(D7h)	9591	0.1
2400	12	–1 (FFh)	–164 (5Ch)		2398	0.1
1200	12	–2 (FEh)	–72	(B8h)	1199	0.1
19200	11.0592	–1 (FFh)	–18	(EEh)	19200	0
9600	11.0592	–1 (FFh)	–36	(DCh)	9600	0
2400	11.0592	–1 (FFh)	–144(70h)		2400	0
1200	11.0592	–2 (FFh)	–32	(E0h)	1200	0

NOTE: OSC. FREQ. IS DIVIDED BY 2, NOT 12.

CORE 2 CONTROL

TIMER 1

OVERFLOW

2
0	1

SMOD

CLK	C/T2 = 0
T2	C/T2 = 1
PIN
	TR2
NOTE: AVAILABILITY OF ADDITIONAL
EXTERNAL INTERRUPT

		TIMER 2
TL2	TH2	OVERFLOW 1	0
(8-BITS)	(8-BITS)
		1	0
		RELOAD
RCAP2L	RCAP2H

RCLK

16

TCLK

16

RX CLOCK

TX CLOCK

T2EX	EXF
PIN	2

CONTROL

TRANSITION

DETECTOR

EXEN2

TIMER 2 INTERRUPT

Figure 34. Timer 2, UART Baud Rates

REV. B –37–