Intel386™ EX EMBEDDED MICROPROCESSOR USER’S MANUAL
13-6
The baud-rate generator contains a seven-bit down counter. A programmable baud-rate value
(BV) is the reload value for the counter. The counter counts down from BV to zero, toggles the
baud-rate generator output, then reloads the BV and counts down again. The baud-rate genera-
tor’s output is a function of BV and BCLKIN as follows.
A BV of 0 gives the maximum output frequency (BCLKIN/2) and a BV of 7FH (127) gives the
minimum output frequency (BCLKIN/256).
If you know the desired baud-rate output frequency, you can determine BV as follows.
The maximum and minimum baud-rate output frequencies with a 33M Hz (CLK2 = 66MHz) de-
vice are shown in Table 13-2.
13.2.2 Transmitter
The transmitter contains a 16-bit buffer and a 16-bit shift register. When the transmitter is en-
abled, the contents of the buffer are immediately transferred to the shift register. The shift register
shifts data out via SSIOTX. Either the internal baud-rate generator (master mode) or an input sig-
nal on the STXCLK pin (slave mode) drives the transmitter. The maximum transmitter inp ut fre-
quency is 8.25 MHz with a 33MHz processor clock (CLK2 = 66MHz). In master mode, the baud-
rate generator must be programmed and enabled prior to enabling the transmitter. In slave mode,
the transmitter must be enabled prior to the applica tion of an external clock.
Table 13-2. Maximum and Minimum Baud-rate Output Frequencies
Baud-rate Value
(BV) Input Frequency (BCLKIN) Output Frequency
0 16.5 MHz
(using either SERCLK or PSCLK wit h a
prescale value of 0)
8.25 MHz
7FH 64.327 KHz
(using PSCLK with a prescale valu e of
1FFH)
251.277 Hz
baud-rate output fre quency BCLKIN
2BV 2+
----------------------=
BV BCLKIN
2 baud-rate output frequency×
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