Intel 386 - Manual (Page 405)

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 33MHz (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 input 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 application 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 with a
prescale value of 0) 8.25 MHz
7FH 64.327 KHz
(using PSCLK with a prescale value of
1FFH) 251.277 Hz
baud-rate output frequency
BCLKIN
2BV 2 +
---------------------- =
BV
BCLKIN
2 baud-rate output frequency Ч
---------------------------------------------------------------------------------  
 
1 – =