T83C5121 datasheet(30/110 Pages) ATMEL

30 / 110 page

T8xC5121

4164E–SCR–02/04

Waiting Time Counter (WT)

The WT counter is a 24 bits down counter which can be loaded with the value contained

in the SCWT2, SCWT1, SCWT0 registers. Its main purpose is time out signal genera-

tion. It is 24 bits wide and is decremented at the ETU rate. The ETU counter acts as a

prescaler (See Figure 15).

When the WT counter timeout, an interrupt is generated and the SCIB function is

locked: reception and emission are disabled. It can be enabled by resetting the macro or

reloading the counter.

Figure 15. Waiting Time Counter

The counter is loaded, if WTEN = 0, during the write of SCWT2 register.

This counter is available in both UART and manual modes. But the behaviour depends

on the selected mode.

In manual mode, the WTEN signal controls the start of the counter (rising edge) and the

stop of the counter (falling edge). After a time out of the counter, a falling edge on

WTEN, a reload of SCWT2 and a rising edge of WTEN are necessary to start again the

counter and to release the SCIB macro. The reload of SCWT2 transfers all SCWT0,

SCWT1 and SCWT2 registers to the WT counter.

In UART mode there is an automatic load on the start bit detection. This automatic load

is very useful for changing on-the-fly the Timeout value since there is a register to hold

the load value. This is the case, for example, when in T = 1 a launch is performed on the

BWT Timeout on the start bit of the last transmitted character. But on the receipt of the

first character an other time out value (CWT) must be used . For this, the new load value

of the waiting time counter must be loaded with CWT before the transmission of the last

character. The reload of SCWT[2-0] with the new value occurs with WTEN = 1.

After a time out of the counter in UART mode, the restart is done as in manual mode.

The maximum interval between the start leading edge of a character and the start lead-

ing edge of the next character is loaded in the SCWT2, SCWT1, SCWT0 registers.

In T = 1 mode, the CWT (character waiting time) or the BWT (block waiting time) are

loaded in the same registers.

The maximum time between two consecutive start bit is WT[23:0] * ETU.

When used to check BWT according to ISO 7816, WT can be set between 971 and

15728651.

ETU Counter

WT Counter

Timeout

SCWT2

SCWT1

SCWT0

WT[23:0]

Load

WTEN

Start bit

UART

Write_SCWT2

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