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MPC8309CVMAGDCA 用户编程技术手册 - NXP（恩智浦）
制造商:
NXP（恩智浦）
分类:
微处理器
封装:
BGA-489
描述:
NXP MPC8309CVMAGDCA 芯片, 微处理器, 32位, 400MHZ, MAPBGA-489
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MPC8309CVMAGDCA数据手册
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PROFIBUS on the QUICC Engine Block, Rev. 1
4 Freescale Semiconductor
Baud rate generation and timers
PROFIBUS is required to support the following baud rates:
• 12 M, 6 M, 3 M, 1.5 M, 500 k, 187.5 k, 93.75 k, 45.45 k, 19.2 k, 9.6 k
Commonly used baud rates less than 100 k are not supported using the CETSCR prescaler directly. There
is not sufficient granularity to achieve a timer output with a value equal to 1 bit time at these lower baud
rates. As a result, the microcode must provide the increased granularity.
The PROFIBUS microcode supports a scaling_factor parameter for scaling the 12 MHz output. This is
used to calculate the time-out values for the IEC timers. For example, if the system uses a baud rate of 1.5
M, then the scaling_factor is 12 M / 1.5 M = 8. The number of bits for an IEC timer is multiplied by the
scaling_factor parameter to augment the CETSCR output to a value coherent with the baud rate being used.
Taking the previous example where the minimum T
SDR
is 11 bits, the microcode waits 88
(11*scaling_factor) increments of CETSCR before declaring this timer has expired at 1.5 Mbaud.
Recommended best practice: Program the CETSCR[CETPS1] to output a 12 MHz clock so each
increment is 1 bit time at 12 Mbaud.
1.3.1 Recommended operating frequencies
Because CETSCR must be used for IEC timers, the QUICC Engine clock must always be a multiple of the
baud rate. Otherwise, IEC timing functions could be inaccurate.
The recommended operating frequencies for QUICC Engine block in PROFIBUS mode are:
• 192 MHz
• 288 MHz
• 384 MHz
If the QUICC Engine clock is 192 MHz or 384 MHz, the lowest baud rate supported using
CETPS1[TimerPrescale] is 187.5 kbaud or 375 kbaud, respectively.
These QUICC Engine clock frequencies work well with all of the commonly used PROFIBUS baud rates.
They also work well for the UART’s physical oversampling rate that is programmed in the BRG register.
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