I now have an urgent (been waiting for the 40X2) application that needs six ADC channels and would like to know if it is possible to enable any of the 28X1 additional analog inputs (AN8-AN13) shown on the PIC16F886 data sheet? It seems quite easy to enable additional analog channels on the 14M and -not so easily- on the 18X. I have not been able to find any documentation about doing this on the 28 or 40X1 and would very much appreciate any relevant information.
Additional 28/40X1 ADC channels?
- Thread starter MFB
- Start date
westaust55
Moderator
I do not think that it is posible with the 28X1 or 40X1 to access the additional pins with Analogue capabilities.
To change a pin to an analogue function, you need access to the ANSEL and/or ANSELH special function registers (SFR's) but the registers required ($188 and $189) are outside the range of the PEEK and POKE commands (range = $0 to $0FF)
To change a pin to an analogue function, you need access to the ANSEL and/or ANSELH special function registers (SFR's) but the registers required ($188 and $189) are outside the range of the PEEK and POKE commands (range = $0 to $0FF)
But, the 40X1 does have 7 analogue inputs, so should work, if you only need 6. For what it is worth, WES, now AMC (the US distributor) has a nifty looking I2C board with, I seem to recall, 8 each 12 bit ADC channels for under USD$20. http://world-educational-services.net/cart/index.php?main_page=product_info&cPath=98&products_id=376&zenid=f7b15009a99a2e1e85e0c406bf0bc3b3
Cheers,
Wreno
Cheers,
Wreno
westaust55
Moderator
Untested but a scheme similar to the attached was proposed recently.
If the analogue device load is only a few milli-amps then you may net even need the transistor switches.
There may be some degradation of the signals due to some loading of the unpowered device. In fact, some degradation is likely its more a case of how much.
If you wish to stick with your 28X1 with its 5 Analog inputs then this may be worth a try.
If the analogue device load is only a few milli-amps then you may net even need the transistor switches.
There may be some degradation of the signals due to some loading of the unpowered device. In fact, some degradation is likely its more a case of how much.
If you wish to stick with your 28X1 with its 5 Analog inputs then this may be worth a try.
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Adding a mux
For what its worth, the approach that I have used in the past, to provide additional analog channels, is to place a MAX4639 dual 4:1 multiplexer in front of the four 28X1 ADC inputs. By connecting the multiplexer’s four control lines together and toggling them from a PICAXE output it is possible to rapidly scan eight analog input channels, in banks of four.
An alternative would have been to use a MAX4638 single 8:1 multiplexer but this would have needed three PICAXE outputs and been slower to scan. However, it would free another three ADC inputs to give a total of 11 analog channels. Either way, it needs an additional chip compared with using a n(admittedly larger) PICAXE 40X1.
For what its worth, the approach that I have used in the past, to provide additional analog channels, is to place a MAX4639 dual 4:1 multiplexer in front of the four 28X1 ADC inputs. By connecting the multiplexer’s four control lines together and toggling them from a PICAXE output it is possible to rapidly scan eight analog input channels, in banks of four.
An alternative would have been to use a MAX4638 single 8:1 multiplexer but this would have needed three PICAXE outputs and been slower to scan. However, it would free another three ADC inputs to give a total of 11 analog channels. Either way, it needs an additional chip compared with using a n(admittedly larger) PICAXE 40X1.