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pig2vcd is a utility which reads notifications on stdin and writes the output as a Value Change Dump (VCD) file on stdout.

The VCD file can be viewed using GTKWave.


Notifications consist of 12 bytes with the following binary format.

typedef struct
   uint16_t seqno;
   uint16_t flags;
   uint32_t tick;
   uint32_t level;
} gpioReport_t;

seqno: starts at 0 each time the handle is opened and then increments by one for each report.

flags: two flags are defined, PI_NTFY_FLAGS_WDOG and PI_NTFY_FLAGS_ALIVE. If bit 5 is set (PI_NTFY_FLAGS_WDOG) then bits 0-4 of the flags indicate a gpio which has had a watchdog timeout; if bit 6 is set (PI_NTFY_FLAGS_ALIVE) this indicates a keep alive signal on the pipe/socket and is sent once a minute in the absence of other notification activity.

tick: the number of microseconds since system boot. It wraps around after 1h12m.

level: indicates the level of each gpio. If bit 1<<x is set then gpio x is high. pig2vcd takes these notifications and outputs a text format VCD.

VCD format

The VCD starts with a header.

$date 2013-05-31 18:49:36 $end
$version pig2vcd V1 $end
$timescale 1 us $end
$scope module top $end
$var wire 1 A 0 $end
$var wire 1 B 1 $end
$var wire 1 C 2 $end
$var wire 1 D 3 $end
$var wire 1 E 4 $end
$var wire 1 F 5 $end
$var wire 1 G 6 $end
$var wire 1 H 7 $end
$var wire 1 I 8 $end
$var wire 1 J 9 $end
$var wire 1 K 10 $end
$var wire 1 L 11 $end
$var wire 1 M 12 $end
$var wire 1 N 13 $end
$var wire 1 O 14 $end
$var wire 1 P 15 $end
$var wire 1 Q 16 $end
$var wire 1 R 17 $end
$var wire 1 S 18 $end
$var wire 1 T 19 $end
$var wire 1 U 20 $end
$var wire 1 V 21 $end
$var wire 1 W 22 $end
$var wire 1 X 23 $end
$var wire 1 Y 24 $end
$var wire 1 Z 25 $end
$var wire 1 a 26 $end
$var wire 1 b 27 $end
$var wire 1 c 28 $end
$var wire 1 d 29 $end
$var wire 1 e 30 $end
$var wire 1 f 31 $end
$upscope $end
$enddefinitions $end

The header defines gpio identifiers and their name. Each gpio identifier must be unique. pig2vcd arbitrarily uses 'A' through 'Z' for gpios 0 through 25, and 'a' through 'f' for gpios 26 through 31. The corresponding names are 0 through 31.

The VCD file may be edited to give a frendlier name, e.g. 8 could be changed to ENCODER_A if an encoder switch A is connected to gpio 8.

Following the header pig2vcd takes notifications and outputs a timestamp followed by a list of one or more gpios which have changed state. The timestamp consists of a '#' followed by the microsecond tick. The state lines contain the new state followed by the gpio identifier.

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Updated: 23/03/2019