TMP92FD54AI datasheet(47/84 Pages) TOSHIBA

47 / 84 page

2006-01-27

92FD54AI-47

TMP92FD54AI

6) Determination of Serial Operation mode

When communicating in UART Mode, the controller should transmit the data value

86H as the first byte at the desired baud rate.

Figure 4.3.3 Data for determining the serial operation mode

The boot program disables reception of the serial operation method recognition data

(86H) received from the controller as the first byte after a Reset and finds the

durations of tAB, tAC and tAD shown in Figure 4.3.3 using the procedure shown in the

flowchart in Figure 4.3.4. As shown in the flowchart in Figure 4.3.4, when the CPU

detects a level change on the receive pin while monitoring the pin status, it latches the

current timer value. For this reason, the timer values for tAB, tAC and tAD are

somewhat erroneous. Also, if the baud rate is fast, the CPU may not be able to detect a

level change on the receive pin.

As shown in the flowchart in Figure 4.3.5, the serial operation mode is determined by

the relative magnitude of the durations tAB and tCD when the receive pin level is Low.

When tAB < tCD, the serial operation mode is found to be UART Mode and the boot

program determines from the duration of tAD whether or not the baud rate can be

automatically set. Since the timer values for tAB, tAC and tAD include some error, as

mentioned earlier, if the baud rate is fast and the operating frequency slow, the timer

values will not be long enough to enable the boot program to determine the serial

operation mode correctly as intended.

For example, it may be erroneously determined that the serial operation mode is not

UART Mode when in fact the controller intends to communicate in UART Mode. To

avoid this kind of problem, if the controller is trying to communicate in UART Mode,

a time-out time after transmission of the first byte of data should be set for the

controller; if the data 86H cannot be received normally from the device within the

time-out time, this can be used to determine that the device cannot communicate.

UART (86H)

tAB

Point A

Point B

Point C

Point D

bit 7

bit 0

bit 1

bit 2

bit 3

bit 4

bit 5

bit 6

Start

Stop

tCD

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