 |
 |
| View previous topic :: View next topic |
| Author |
Message |
kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
|
| I2C multimaster. Switching PIC between Master and Slave. |
 Posted: Sun Jan 13, 2008 8:10 pm |
 |
|
Colleagues,
Is it possible to choose between Master and Slave modes on the fly by writing to the SSPCON register?
I will have two PICs in the system. One will handle the display, another one will control the process. When the process isn’t running, the display PIC should be the Master. When the process is running, the display PIC should display the parameters. But the problem is that I have a requirement from the guy, who developed the current version of the control PIC firmware, that the control PIC shouldn’t be interrupted for serial communication. I’m contemplating about making a multimaster bus, where the process control PIC is a Master, when the process is running, and the display PIC is a Master for the rest of the time. I’ll probably add a handshaking line too. Do you think that this is a good approach?
Another option would be to replace I2C with CAN, which is a peer-to-peer bus to begin with.
Cheers,
- Nick
_________________
Read the label, before opening a can of worms. |
|
 |
kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
|
|
| Posted: Sun Mar 09, 2008 4:56 pm |
|
|
Here's a code library thread on a similar subject. http://www.ccsinfo.com/forum/viewtopic.php?t=33950
It shows how to configure a PIC as a hardware slave on one pair of pins (SCL & SDA) and a software master on another pair. These two connections can go to the same I2C bus, and as a result the PIC can act as both master and slave on the same bus (at the expense of committing another pair pins).
_________________
Read the label, before opening a can of worms. |
|
|
kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
|
| Compiler acts strange, when it sees a second #use i2c |
| Posted: Sat Sep 06, 2008 4:06 pm |
|
|
Revisiting this area. First, I've looked at the assembly code generated by the #use i2c directive and found some instructions that configure the SSP control registers. Then I thought - may be #use i2c directive can be used multiple times to reconfigure the SSP peripheral. I tried that, but the compiler acted strangely.
Here's my test code:
| Code: |
while (1)
{
#use i2c(Master, Fast, sda=PIN_C4, scl=PIN_C3, force_hw) // become a master temporarily
// dummy i2c transaction
i2c_start();
i2c_write(0xA0);
i2c_stop();
#use i2c(Slave, sda=PIN_C4, scl=PIN_C3, address=0xA0, FORCE_HW, NO_STRETCH) // become a slave again
delay_ms(1000);
output_toggle(POWER_LED); // show lifesigns
}
|
Here's the assembly listing:
| Code: |
.................... while (1)
.................... {
.................... #use i2c(Master, Fast, sda=PIN_C4, scl=PIN_C3, force_hw) // become a master for
*
00CE: BCF FC6.7
00D0: BCF F9E.3
00D2: MOVFF 1A,FC9
00D6: MOVLW 02
00D8: BTFSC FC6.7
00DA: BRA 00E6
00DC: BTFSS F9E.3
00DE: BRA 00DC
00E0: MOVLW 00
00E2: BTFSC FC5.6
00E4: MOVLW 01
00E6: MOVWF 01
00E8: GOTO 017A (RETURN)
.................... i2c_start();
*
016E: BSF FC5.0
0170: BTFSC FC5.0
0172: BRA 0170
.................... i2c_write(0xA0);
0174: MOVLW A0
0176: MOVWF 1A
0178: BRA 00CE
.................... i2c_stop();
017A: BSF FC5.2
017C: BTFSC FC5.2
017E: BRA 017C
.................... #use i2c(Slave, sda=PIN_C4, scl=PIN_C3, address=0xA0, FORCE_HW, NO_STRETCH) // become a slave again
....................
....................
.................... delay_ms(1000);
0180: MOVLW 04
0182: MOVWF 1A
0184: MOVLW FA
0186: MOVWF 1B
0188: RCALL 00A8
018A: DECFSZ 1A,F
018C: BRA 0184
.................... output_toggle(POWER_LED); // blink
018E: BCF F96.2
0190: BTG F8D.2
.................... }
|
As you can see, the compiler had ignored the second #use i2c. It didn't generate an error or a warning. When I ran my code, it didn't execute past the i2c_stop() line. The PIC froze there.
When #use i2c(slave,...) is replaced with #use i2c(master,...) the compiler still ignores it, but the PIC doesn't freeze.
PCWH 4.071, PIC18F4525
_________________
Read the label, before opening a can of worms. |
|
|
kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
|
| ... and that was only a beginning. |
| Posted: Sat Sep 06, 2008 11:29 pm |
|
|
Here's a more fundamental challenge. #use i2c directive defines the i2c_start(), i2c_stop(), i2c_read() and i2c_write(). The definitions are different for master and slave modes:
| Code: | #use i2c(Master, Fast, sda=PIN_C4, scl=PIN_C3, force_hw)
.................... i2c_start();
020E: BSF FC5.0
0210: BTFSC FC5.0
0212: BRA 0210
.................... i2c_write(0xA0);
0214: MOVLW A0
0216: MOVWF 1A
0218: BRA 00CE
.................... i = i2c_read();
021A: MOVLW 01
021C: MOVWF 00
021E: BRA 00EC
0220: MOVFF 01,19
.................... i2c_stop();
0224: BSF FC5.2
0226: BTFSC FC5.2
0228: BRA 0226 |
| Code: | #use i2c(Slave, sda=PIN_C4, scl=PIN_C3, address=0xA0, NO_STRETCH)
.................... i2c_start();
01E8: BSF F94.3
01EA: BSF F94.4
01EC: BTFSS F82.4
01EE: BRA 01EC
01F0: BTFSS F82.3
01F2: BRA 01F0
01F4: BTFSC F82.3
01F6: BRA 01F0
01F8: BCF F94.3
01FA: BCF F8B.3
.................... i2c_write(0xA0);
01FC: MOVLW A0
01FE: MOVWF 1A
0200: BRA 00CE
.................... i = i2c_read();
0202: BTFSS FC7.0
0204: BRA 0202
0206: MOVFF FC9,19
.................... i2c_stop();
020A: BSF F94.3
020C: BSF F94.4
|
So, to have a PIC that can switch between being a master and a slave, one should have two separate sets of read and write routines (e.g. i2c_slave_read(), ...).
BTW, what would i2c_start() in slave mode do? Should an I2C slave ever start a transaction?
_________________
Read the label, before opening a can of worms. |
|
|
FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
|
|
| Posted: Sun Sep 07, 2008 3:02 am |
|
|
| I wouldn't except, that the CCS built-in functions provide the discussed functionality of alternative master and slave operation. But the underlying PIC hardware is capable of it without doubt. So it is available by direct register programming. |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
FAQ 
Search
Register
Profile 
Log in to check your private messages 
Log in
