P80CL580 Datasheet, PDF (36/80 Pages) NXP Semiconductors – Low voltage 8-bit microcontrollers with UART, I2C-bus and ADC

Philips Semiconductors
Low voltage 8-bit microcontrollers with
UART, I²C-bus and ADC
Product specification
P80CL580; P83CL580
16.3.2 USING TIMER 2 TO GENERATE BAUD RATES
Timer 2 is selected as a Baud Rate Generator by setting
the RTCLK bit in T2CON. The Baud Rate Generator mode
is similar to the Auto-reload mode, in that a roll-over in TH2
causes Timer 2 registers to be reloaded with the 16-bit
value held in the registers RCAP2H and RCAP2L, which
are preset by software. Baud rates in Modes 1 and 3 are
determined by Timer 2's overflow rate as specified below.
Baud Rate = ^{T----i-m-----e----r---2-----O-----1v---e6---r--f--l-o----w-----R-----a---t--e--}
The Timer 2 can be configured for either ‘timer’ or ‘counter’
operation. In the most typical applications, it is configured
for ‘timer’ operation (C/T2 = 0). ‘Timer’ operation is slightly
different for Timer 2 when it is being used as a Baud Rate
Generator. Normally, as a timer it would increment every
machine cycle at a frequency of ¹â„₁₂ × f_osc. However, as a
Baud Rate Generator it increments every state time at a
frequency of ¹â„₂ × f_osc. In this case the baud rate in
Modes 1 and 3 is determined as:
Baud Rate = _{3----2-----×}-----_{----6---5---5----3---6-----_–-----₍---R---f-C-^o--^s-A-^c---P----2----H----;---R-----C----A----P----2---L----₎---_}--
Where (RCAP2H; RCAP2L) is the content of registers
RCAP2H and RCAP2L taken as a 16-bit unsigned integer.
The Baud Rate Generator mode for Timer 2 is shown in
Fig.18. This figure is only valid if RTCLK = 1. At roll-over
TH2 does not set the TF2 bit in T2CON and therefore, will
not generate an interrupt. Consequently, the Timer 2
interrupt does not need to be disabled when in the Baud
Rate Generator mode. If EXEN2 is set, a HIGH-to-LOW
transition on T2EX will set the EXF2 bit, also in T2CON,
but will not cause a reload from (RCAP2H; RCAP2L) to
(TH2, TL2). Therefore, in this mode T2EX may be used as
an additional external interrupt.
When Timer 2 is operating as a timer (TR2 = 1), in the
Baud Rate Generator mode, registers TH2 and TL2 should
not be accessed (read or write). Under these conditions
the timer is being incremented every state time and
therefore the results of a read or write may not be
accurate. The registers RCAP2H and RCAP2L however,
may be read but not written to. A write might overlap a
reload and cause write and/or reload errors. If a write
operation is required, Timer 2 or RCAP2H/RCAP2L
should first be turned off by clearing the TR2 bit.
handbook, full pagewidth
(note: divided by 2
not by 12)
OSC
2
C/T2 = 0
T2 PIN
C/T2 = 1
transition
detector
T2EX PIN
control
TR2
TIMER 1
overflow
2
0
1
TL2
(8 BITS)
TH2
(8 BITS)
1
0
SMOD
RTCLK
RELOAD
RCAP2L RCAP2H
16
CLK
UART receive/
transmit clock
control
EXEN2
EXF2
TIMER 2 interrupt
(additional external interrupt)
MGD622
Fig.18 Timer 2 in Baud Rate Generator mode.
1997 Mar 14
36