Rev. D - 06 december, 2000

9

TS87C51U2

Figure 2. Mode Switching Waveforms

The X2 bit in the CKCON register (See Table 4.) allows to switch from 12 clock cycles per instruction to 6 clock

cycles and vice versa. At reset, the standard speed is activated (STD mode). Setting this bit activates the X2 feature

(X2 mode).

CAUTION

In order to prevent any incorrect operation while operating in X2 mode, user must be aware that all peripherals

using clock frequency as time reference (UARTs, timers) will have their time reference divided by two. For example

a free running timer generating an interrupt every 20 ms will then generate an interrupt every 10 ms. UART with

4800 baud rate will have 9600 baud rate.

XTAL1:2

XTAL1

CPU clock

X2 bit

X2 Mode

STD Mode

STD Mode