AM85C30 datasheet(136/194 Pages) AMD

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AMD

6–26

6.2.13

Write Register 12 (Lower Byte of Baud Rate

Generator Time Constant)

WR12 contains the lower byte of the time constant for the baud rate generator. The time

constant can be changed at any time, but the new value does not take effect until the next

time the time constant is loaded into the down counter. No attempt is made to synchro-

nize the loading of the time constant into WR12 and WR13 with the clock driving the

down counter. For this reason, it is advisable to disable the baud rate generator while the

new time constant is loaded into WR12 and WR13. Ordinarily, this is done anyway to pre-

vent a load of the down counter between the writing of the upper and lower bytes of the

time constant.

The formula for determining the appropriate time constant for a given baud is shown be-

low with the desired rate in bits per second and the BR clock period in seconds. This for-

mula is derived because the counter decrements from N down to ‘0’-plus-one-cycle for

reloading the time constant and is then fed to a toggle flip-flop to make the output a

square wave. Bit positions for WR12 are shown in Figure 6–14.

Time Constant =

• (Desired Rate) • (Baud Rate)

–2

D7 D6 D5 D4 D3 D2 D1 D0

TC3

TC4

TC5

TC6

TC0

TC1

TC2

TC7

Lower Byte of

Time Constant

Figure 6–14. Write Register 12

6.2.14

Write Register 13 (Upper Byte of Baud Rate

Generator Time Constant)

WR13 contains the upper byte of the time constant for the baud rate generator. Bit posi-

tions for WR13 are shown in Figure 6–15.

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