how to write the drivers

How to write device drivers is a big topic. The best place to start for eCos (in fact the best place to start for writing any code for eCos) is the eCos reference manual that is supplied with the eCos installer. Information on how device drivers are used and how to write them can be found in the section "I/O Package (Device Drivers)", in particular the section "How to write a Driver" would be a good starting point.

In your case, a simple character mode device driver may be apropriate. If so, then all you need to do is use the CHAR_DEVIO_TABLE and CHAR_DEVTAB_ENTRY macros (and supply the required functions) to create your device. For example:

CHAR_DEVIO_TABLE(my_adc_handlers, 
                 NULL,                                
                 my_adc_read, 
                 NULL,                  
                 NULL,            
                 NULL);       
                                                  
  CHAR_DEVTAB_ENTRY(my_adc_device,     
                    "/dev/adc",          
                    NULL,                 
                    &my_adc_handlers,    
                    my_adc_init,          
                    my_adc_lookup,      
                    NULL);    

If you search through the eCos source, you'll find a number of mouse/touchscreen drivers that use the character device model. These may serve as useful examples.

Of course, this is probably overkill. You may find it more apropriate to simply write a function as a part of your application that reads from your device directly.