Serial Wombat
a general-purpose digital interface
device for hobbyists, engineers and students
Did you know...
Serial Wombat Datalogger Pin Mode
The Serial Wombat can store any pin's public data into a queue at regular intervals. This allows much more precise sampling of data than would be possible through queries over the serial port. For instance, one might wish to sample one of the analog-to-digital conversion pins every 5ms and store the data into a queue in the user ram area. Since the queues can be filled by the pin mode and read by serial commands in real time it is possible to achieve a stream of periodically sampled data to the host, given a sufficently slow sampling frequency and sufficiently high baud rate.
The pin assigned to datalogger mode can be a pin which is being sampled, or can be a control pin which controls sampling or outputs sample status. See the examples below for more detail.
The Datalogger mode has a number of options:
- Pin function: The pin which is assigned to datalogger mode may be configured to be a signal input, a control input, or a control output. Signal inputs are sampled for raw A/D value. Control inputs can be used to gate the sampling of other pins' public data (for instance only sample some pin(s) while the datalogger pin is high), or as a trigger (for instance, begin sampling some pin(s) once the datalogger pin goes low). The datalogger pin can also be a control output. It can be configured to go high while sampling is occuring, then go low once the queue has filled.
- Pins to be sampled: When a pin is configured to datalogger mode the data from up to 4 pins can be pushed into the queue at a time. The pin which is configured to datalogger mode is not necessarily the pin whose data will be stored in the queue. For instance, pin 20 might be configured as a datalogger which stores data from pins 2,3,4 and 5.
- Frames between samples: The number of frames (by default a frame is 1 ms) which elapse between storing values to the queue. This value can range between 0 (store on every frame) and 255 (store on every 256th frame).
- Sample Resolution: The datalogger can be
configured to store all 16bits of the sampled pins'
public data, only the most significant 8 bits, or only
the least significant 8 bits. This allows the user to
choose how much storage is appropriate for his or her
data.
For instance, in an application where precision of A/D conversion was most important, all 16 bits of an A/D pin might be stored. However, only 10 bits of this data is really useful, since the hardware A/D converter resolution is only 10 bits. A user might choose instead to only store the 8 most significant bits in order to effectively double the number of stored samples in exchange for loosing 2 bits of resolution.
In a case where the user was sampling the public data of a pin mode whose output was always less than 256 then choosing only the least significant 8 bits would make sense. - Queue Address: The pin mode puts data into a queue in User ram. The host is responsible for initializing this queue and providing the address of the queue to the datalogger pin mode.
- Queue Overflow action: The datalogger can deal with a full queue in two ways: it can stop logging data, or it can throw out the oldest piece of data in order to make room for a new one.
Message format:
Message 200 is used to initialize the Pin mode. It tells the pin mode which pins should be logged to the queue (up to 4 pins), and how long to wait between samples.
| Data Sent to the Wombat: | 200 | Pin # | 31 | Pin to log | Pin to log | Pin to log | Pin to log | Delay between samples |
| Meaning: | Configure Pin First message | Pin Number to be set to Data Logger Mode | Data Logger Mode | First of up to 4 pins to log to queue. 255 if unused. | Second of up to 4 pins to log to queue. 255 if unused. | Third of up to 4 pins to log to queue. 255 if unused. | Fourth of up to 4 pins to log. 255 if unused | Number of frames to wait between logging data |
| This message is echoed back by the Wombat. | ||||||||
Message 201 is used to configure how the datalogger pin behaves, and how to deal with a full queue.
| Data Sent to the Wombat: | 201 | Pin # | 31 | Data Logger Pin Function | Active State: | Inactive State | Full queue action | 0x55 |
| Meaning: | Configure Pin Second message | Pin Number to be set to Data Logger Mode | Data Logger Mode | 0: Fixed. Used if the pin is
one of the pins being sampled. 1: Enable Gate input. The datalogger will only log data when this pin reads a digital value equal to the 'active' setting. 2: Trigger input. The datalogger will begin logging when the active state is detected, and continue logging after that point. 3: Status Output: The pin will display the 'Active' state while logging, and 'Inactive' when not logging (usually because of full buffer). |
In Enable Gate and Trigger
pin mode the digital value which corresponds to logging
(0 or 1). In Status output mode, the pin state to indicate currently logging (0: low, 1: high, 2: high impedance). Unused in Fixed mode. |
Used in Status output mode for the pin state when not logging. (0: low, 1: high, 2: high impedance). | 0: Stop logging if queue
fills up. 1: Discard oldest data if queue fills up. |
Unused. |
| This message is echoed back by the Wombat. | ||||||||
Message 202 is used to configure the queue address and set the initial mode of the data logger.
| Data Sent to the Wombat: | 202 | Pin # | 31 | Bytes of Public Data to Log | Initial State | Queue Address High Byte | Queue Address Low Byte | 0x55 | |
| Meaning: | Configure Pin Third message | Pin Number to be set to Data Logger Mode | Data Logger Mode | 0: all 16 bits 1: High 8 bits 2: Low 8 bits |
1: Waiting for trigger (used
with triggered pin input setting) 2: Logging: Start logging on receipt of this message |
Address of queue into which to place data. Queue should be initialized by the host before issuing this command. | Unused. | ||
| This message is echoed back by the Wombat. | |||||||||
Note: The size of the queue used to store the logged data should be an even multiple of the number of bytes stored each time data is logged. For instance, if all 16 bits of one pin's data is being logged, then the queue size should be a multiple of 2. If the least significant 8 bits from three different pins is being logged, then the queue size should be a multiple of 3. If all 16 bits of 4 pins are being logged, then the queue size should be a multiple of 8.
Example:
A pressure sensor is attached to pin number 2. That pin has
been configured to perform
a/d conversion and averaging of 16 frames:
200 2 15 4 0x55 0x55 0x55 0x55
; Pin 2 is configured for average mode (15)
; Averaging 16 samples ( 2 to the 4th power)
A queue is created at address 0x100 in user ram. The queue size will be 200 bytes:
128 0x100 0 200 0 0x55 0x55 0x55
Configure a pin to datalogger mode. Since we'll be logging the output of another pin and aren't actually inputing or outputing physical signals with the datalogger itself, we'll use one of the Wombat's power pins, number 11. Since the averaging mode on pin 2 will only produce a new reading every 16 frames, we'll configure the data logger to log every 16 frames.
200 11 31 2 255 255 255 15
; Configure pin 11 to mode 31 (datalogger)
; Log pin 2, no other pins, skip 15 frames between logs
201 11 31 0 0 0 0 0x55
; Fixed mode ( 0 ), since we aren't using pin 15 for I/O
; The next two bytes don't matter, since they don't apply to fixed
; The final 0 means stop if the queue fills up we'll stop logging
;
202 11 31 0 2 0x0100 0x55; Log 16 bits per sample. Set queue address to 0x0100, and start logging.
This example will push one 16-bit value into the queue every 16 frames. Assuming the frame time is 1ms, this will generate about 125 bytes of data per second. Using the Get Queue Data command up to 6 bytes of data can be retreived per 16 byte transaction. Therefore, the host will need to average at least 21 Get Queue Data requests per second. 21 transactions times 16 bytes per transaction means 336 bytes per second must be transferred. With 1 start bit, 8 data bits, and 1 stop bit per byte, this means 3360 bits per second. Therefore a baud rate of 4800 bps should be sufficent to keep up with the queue. 9600 baud or higher should assure that the host can keep the queue drained while also doing other things.