參數(shù)資料
型號: 16F872
廠商: Microchip Technology Inc.
英文描述: CAT 5E CROSSOVER PATCH CORD CABLE GREEN 10 FT
中文描述: 28引腳,8位閃存微控制器的CMOS
文件頁數(shù): 81/160頁
文件大?。?/td> 2600K
代理商: 16F872
1999 Microchip Technology Inc.
Preliminary
DS30221A-page 81
PIC16F872
9.2.18.16 BUS COLLISION DURING A REPEATED
START CONDITION
During a Repeated Start condition, a bus collision
occurs if:
a)
A low level is sampled on SDA when SCL goes
from low level to high level.
b)
SCL goes low before SDA is asserted low, indi-
cating that another master is attempting to trans-
mit a data ’1’.
When the user deasserts SDA and the pin is allowed to
float high, the BRG is loaded with SSPADD<6:0> and
counts down to 0. The SCL pin is then deasserted, and
when sampled high, the SDA pin is sampled. If SDA is
low, a bus collision has occurred (i.e., another master is
attempting to transmit a data ’0’). If, however, SDA is
sampled high, the BRG is reloaded and begins count-
ing. If SDA goes from high to low before the BRG times
out, no bus collision occurs, because no two masters
can assert SDA at exactly the same time.
If, however, SCL goes from high to low before the BRG
times out and SDA has not already been asserted, a
bus collision occurs. In this case, another master is
attempting to transmit a data ’1’ during the Repeated
Start condition.
If, at the end of the BRG time out, both SCL and SDA
are still high, the SDA pin is driven low, the BRG is
reloaded and begins counting. At the end of the count,
regardless of the status of the SCL pin, the SCL pin is
driven low and the Repeated Start condition is com-
plete (
Figure 9-23
).
FIGURE 9-23: BUS COLLISION DURING A REPEATED START CONDITION (CASE 1)
FIGURE 9-24: BUS COLLISION DURING REPEATED START CONDITION (CASE 2)
SDA
SCL
RSEN
BCLIF
S
SSPIF
Sample SDA when SCL goes high.
If SDA = 0, set BCLIF and release SDA and SCL
Cleared in software
’0’
’0’
’0’
’0’
SDA
SCL
BCLIF
RSEN
S
SSPIF
Interrupt cleared
in software
SCL goes low before SDA,
Set BCLIF. Release SDA and SCL
T
BRG
T
BRG
’0’
’0’
’0’
’0’
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