DS89C420-QCL Datasheet, PDF(115/139 Page) Maxim Integrated Products – Ultra-High-Speed Flash Microcontroller User’s Guide

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DS89C420-QCL Datasheet, PDF (115/139 Pages) Maxim Integrated Products – Ultra-High-Speed Flash Microcontroller User’s Guide

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Ultra-High-Speed Flash

Microcontroller Userâs Guide

Table 12-5. Timer 1 Input Clock Frequency

SYSTEM CLOCK MODE

Crystal multiply mode 4X

Crystal multiply mode 2X

Divide-by-1 (default)

Power-management mode (/1024)

PMR REGISTER BITS

4X/2X, CD1, CD0

100

000

X01, X10

X11

TIMER 1

INPUT CLOCK FREQUENCY

T1MH,T1M = 00

T1MH,T1M = 01

T1MH,T1M = 1X

OSC / 12

OSC / 1

OSC / 0.25

OSC / 12

OSC / 2

OSC / 4

OSC / 0.5

OSC / 1

OSC / 3072

OSC / 1024

Using timer 1 in the 8-bit autoreload mode, serial port baud rates for mode 1 or 3 can be calculated using the formula below.

Modes 1, 3 baud rate =

2SMOD_x

Timer 1 input clock frequency

(256 - TH1)

Number of serial bits /

Number of timer 1 rollovers

Timer 1 rollover

frequency

Timer 1 input clock frequency can be found in the previous table, SMOD_x is the logic state of the baud-rate doubler bit for the asso-

ciated UART, and TH1 is the user assigned timer 1 reload value.

Often, users already know what baud rate is desired and only need to calculate the timer reload value. An equation to calculate the

timer reload value, TH1, is as follows:

2SMOD_x x timer 1 input clock frequency

TH1 = 256 -

32 x 5 baud rate

Note that the 8-bit, autoreload mode for timer 1 is the one most commonly used for serial port applications, but that it can actually be configured in any mode, even as a

counter.

Using Timer 2 for Baud-Rate Generation

To use timer 2 as baud-rate generator for serial port 0, the timer is configured in autoreload mode. Then, either the TCLK or RCLK bit

(or both) are set to a logic 1. TCLK = 1 selects timer 2 as the baud-rate generator for the transmitter and RCLK = 1 selects timer 2 for

the receiver. Thus, serial port 0 can have the transmitter and receiver operating at different baud rates by choosing timer 1 for one data

direction and timer 2 for the other. RCLK and TCLK reside in T2CON.4 and TCON.5, respectively.

Although the timer 2 input clock can be configured similarly to timer 1, it must be placed into a baud-rate generator mode in order to

be used by serial port 0. Setting either RCLK or TCLK to a logic 1 selects timer 2 for baud-rate generation. When this is done, the timer

2 input clock becomes fixed to the oscillator frequency divided by 2. This is compatible with the 80C32. The only exception is when

timer 2 is used for baud-rate generation within power-management mode. For PMM, the system clock (OSC/1024) is used as the input

clock for timer 2. The timer 2 interrupt is automatically disabled when either RCLK or TCLK is set. Also, the TF2 (TCON.7) flag is not

set on a timer rollover. The manual reload pin, T2EX (P1.1), does not cause a reload either. Table 12-6 illustrates this relationship.

Table 12-6. Timer 2 Baud-Rate Generation