
Philips Semiconductors
Preliminary specification
87LPC769
Low power, low price, low pin count (20 pin)
microcontroller with 4 kB OTP 8-bit A/D, and DAC
2000 Apr 26
38
Baud Rates
The baud rate in Mode 0 is fixed: Mode 0 Baud Rate = CPU clock/6.
The baud rate in Mode 2 depends on the value of bit SMOD1 in
Special Function Register PCON. If SMOD1 = 0 (which is the value
on reset), the baud rate is 1/32 of the CPU clock frequency. If
SMOD1 = 1, the baud rate is 1/16 of the CPU clock frequency.
Mode 2 Baud Rate
1
SMOD1
32
CPU clock frequency
Using Timer 1 to Generate 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 SMOD1. The Timer 1 interrupt should be disabled in this
application. The Timer itself can be configured for either “timer” or
“counter” operation, and in any of its 3 running modes. In the most
typical applications, it is configured for “timer” operation, in the
auto-reload mode (high nibble of TMOD = 0010b). In that case the
baud rate is given by the formula:
Mode 1, 3 Baud Rate
CPU clock frequency
192 (or 96 if SMOD1
1)
256
(TH1)
Tables 6 and 7 list various commonly used baud rates and how they
can be obtained using Timer 1 as the baud rate generator.
Table 10. Baud Rates, Timer Values, and CPU Clock Frequencies for SMOD1 = 0
Timer Co nt
Timer Count
Baud Rate
9600
2400
4800
19.2k
38.4k
–1
0.4608
0.9216
* 1.8432
* 3.6864
* 7.3728
–2
0.9216
1.8432
* 3.6864
* 7.3728
–
–3
1.3824
2.7648
5.5296
–
–
–4
* 1.8432
* 3.6864
* 7.3728
–
–
–5
2.3040
4.6080
9.2160
–
–
–6
2.7648
5.5296
–
–
–
–7
3.2256
6.4512
–
–
–
–8
* 3.6864
* 7.3728
–
–
–
–9
4.1472
8.2944
–
–
–
–10
4.6080
9.2160
–
–
–