Zilog Z16C35 - Manual (Page 120)

ISCC
User Manual
UM011002-0808
114
If the XTAL oscillator output is programmed to be echoed, and the XTAL oscillator has
not been enabled, the /TRxC pin goes High. The DPLL signal that is echoed is the DPLL
signal used by the receiver. Hardware reset selects the XTAL oscillator as the output
source.
5.4.13 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 synchronize
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 prevent
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
below with the desired rate in bits per second and the BR clock period in seconds. This
formula 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 5-14.
Time constant = [1/2 * desired rate * BR clock period] - 2
Table 5–33. Transmit External Control Selection
Bit 1 Bit 0 TRxC Pin Output
0 0 XTAL Oscillator Output
0 1 Transmit Clock
1 0 BR Output
1 1 DPLL Output (receive)
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