VRS550 Datasheet, PDF (23/40 Pages) List of Unclassifed Manufacturers – VRS550 - 8kB Flash, 256B RAM, 25MHz, 8-Bit MCU VRS560 - 16kB Flash, 256B RAM, 25MHz, 8-Bit MCU

UART Baud Rates Calculation
In Mode 0, the baud rate is fixed and can be
represented by the following formula:
Mode 0 Baud Rate = Oscillator Frequency
12
In Mode 2, the baud rate depends on the value of the
SMOD bit in the PCON SFR. From the formula below,
we can see that if SMOD = 0 (which is the value on
reset), the baud rate is 1/32 the oscillator frequency.
Mode 2 Baud Rate = 2^SMOD x (Oscillator Frequency)
64
The Timer 1 and/or Timer 2 overflow rate determines
the baud rates in modes 1 and 3.
Generating Baud Rate with Timer 1
When Timer 1 functions as a baud rate generator, the
baud rate in modes 1 and 3 are determined by the
Timer 1 overflow rate.
Mode 1, 3 Baud Rate = 2^SMODx Timer 1 Overflow Rate
32
Timer 1 must be configured as an 8-bit timer (TL1) with
auto-reload with TH1 value when an overflow occurs
(Mode 2). In this application, the Timer 1 interrupt
should be disabled.
The two following formulas can be used to calculate
the baud rate and the reload value to put in the TH1
register.
Mode 1, 3 Baud Rate =
2^SMODx Fosc
32 x 12(256 – TH1)
VRS550 / VRS560
VERSA
Datasheet Rev 1.1
The value to put into the TH1 register is defined by the
following formula:
TH1 = 256 -
2^SMODx Fosc
32 x 12x (Baud Rate)
It is possible to use Timer 1 in 16-bit mode to generate
the baud rate for the serial port. To do this, leave the
Timer 1 interrupt enabled, configure the timer to run as
a 16-bit timer (high nibble of TMOD = 0001B), and use
the Timer 1 interrupt to perform a 16-bit software
reload. This can achieve very low baud rates.
Generating Baud Rates with Timer 2
Timer 2 is often preferred to generate the baud rate, as
it can be easily configured to operate as a 16-bit timer
with auto-reload. This allows for much better resolution
than using Timer 1 in 8-bit auto-reload mode.
The baud rate using Timer 2 is defined as:
Mode 1, 3 Baud Rate = Timer 2 Overflow Rate
16
The timer can be configured as either a timer or a
counter in any of its 3 running modes. In most typical
application, it is configured as a timer (C/T2 is set to 0).
To make the Timer 2 operate as a baud rate generator
the TCLK and RCLK bits of the T2CON register must
be set to 1.
The baud rate generator mode is similar to the auto-
reload mode in that an overflow in TH2 causes the
Timer 2 registers to be reloaded with the 16-bit value in
registers RCAP2H and RCAP2L, which are preset by
software. However, when Timer 2 is configured as a
baud rate generator, its clock source is Osc/2.
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