Nxp Semiconductors PCF2123 用户手册

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

When no communication with the RTC is possible, it is also not possible to set and read 
time. Normally the I

2

C-bus will not get stuck, but especially during the development 

phase problems may occur that hang up the bus. One reason may be that spikes on the 
bus lines are interpreted as additional pulses which then would convey data not in line 
with the I

2

C-protocol. Also timing violations when for example two GPIOs are used to 

emulate an I

2

C-bus sometimes lead to unexpected results. On I

2

C-devices a scope may 

be used to verify whether the RTC sends an acknowledgement at the end of each byte. 
Over the past years oscilloscopes have been introduced that allow trigger and decode for 
serial bus protocols, including I

2

C and SPI. Also timing violations can be easily found with 

such equipment. These include rise and fall times, setup time, hold times and also 
voltage levels. 

If the bus gets stuck first it needs to be determined how exactly it is stuck. There are two 
“bus stuck scenarios”: 

•  SCL (clock) stuck low: There is nothing that can be done about this but to hard reset 

the device (remove power) because the I

2

C-bus requires clock edges to clock the 

data; 

•  SDA (data) stuck low, but clock ok: A start condition can’t be sent because this 

requires a high to low transition of SDA while SCL is high. SDA however is stuck low. 
What will work is to send 9 clocks plus a STOP condition. The 9 clock pulses will 
clear the I

2

C state machine, thus causing the device to release the bus. This permits 

the master to send a STOP condition and now the I

2

C interface of the slave will have 

been reset. This works for all I

2

C compatible devices without exception. 

According to the I

2

C specification there exists a so-called general call address. This is for 

addressing every device connected to the I

2

C-bus at the same time. The general call 

address is 00

HEX

. However, if a device does not need any of the data supplied within the 

general call structure, it can ignore this address by not issuing an acknowledgement. An 
I

2

C device does not have to be designed such that it responds to a general call address. 

The real time clocks for which this manual is valid do not respond to the general call 
address. 

Sending the 9 clock pulses with SDA low may seem like sending a general call address 
since data is always zero for every clock pulse. The difference however is that no START 
condition could be sent first since SDA was already stuck low. Sending the 9 clock 
pulses is not the same as sending a general call address. If a device does require data 
from a general call address, it will acknowledge this address and behave as a slave 
receiver. The master does not actually know how many devices acknowledged if one or 
more devices respond. The second and following bytes will be acknowledged by every 
slave-receiver capable of handling this data. A slave which cannot process one of these 
bytes must ignore it by not-acknowledging. These RTCs will not respond to a general 
call. 

In short, when the bus is stuck low due to SDA, the sequence to recover the bus is by 
sending 9 clock pulses plus STOP.  

 

: Only the PCF8593 includes a dedicated  RESET input. When reset occurs only 

the I

2

C-bus interface is reset. Thus for this device a second option of releasing the I

2

C-

bus is available.