brymen-dmm: unbreak dBm reference impedance interpretation

[libsigrok.git] / src / hardware / brymen-dmm / parser.c

diff --git a/src/hardware/brymen-dmm/parser.c b/src/hardware/brymen-dmm/parser.c
index 40ca6c86074084cd6bf320627e1e725c70c6bf15..cc0a05c60c25dd44a0cfb8cca26415b93f22d89e 100644 (file)
--- a/src/hardware/brymen-dmm/parser.c
+++ b/src/hardware/brymen-dmm/parser.c
@@ -223,6 +223,7 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
        uint8_t *bfunc;
        const char *txt;
        int txtlen;
+       char *p;
        char *unit;
        int ret;
 
@@ -237,6 +238,22 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
 
        memset(&flags, 0, sizeof(flags));
        parse_flags(bfunc, &flags);
+       if (flags.is_decibel && flags.is_ohm) {
+               /*
+                * The reference impedance for the dBm function is in an
+                * unexpected format. Naive conversion of non-space chars
+                * gives incorrect results. Isolate the 4..1200 Ohms value
+                * instead, ignore the "0." and exponent parts of the
+                * response text.
+                */
+               if (strncmp(txt, " 0.", strlen(" 0.")) == 0 && strstr(txt, " E")) {
+                       txt = &txt[strlen(" 0.")];
+                       txtlen -= strlen(" 0.");
+                       p = strchr(txt, 'E');
+                       if (p)
+                               *p = '\0';
+               }
+       }
        if (flags.is_fahrenheit || flags.is_celsius) {
                /*
                 * The value text in temperature mode includes the C/F
@@ -264,7 +281,9 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
                analog->meaning->unit = SR_UNIT_AMPERE;
        }
        if (flags.is_ohm) {
-               if (flags.is_beep)
+               if (flags.is_decibel)
+                       analog->meaning->mq = SR_MQ_RESISTANCE;
+               else if (flags.is_beep)
                        analog->meaning->mq = SR_MQ_CONTINUITY;
                else
                        analog->meaning->mq = SR_MQ_RESISTANCE;
@@ -296,20 +315,25 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
        }
 
        /*
-        * The high-end Brymen models have a configurable reference impedance.
-        * When the reference impedance is changed, the DMM sends one packet
-        * with the value of the new reference impedance. Both decibel and ohm
-        * flags are set in this case, so we must be careful to correctly
-        * identify the value as ohm, not dBmW.
+        * The high-end Brymen models have a configurable reference
+        * impedance for dBm measurements. When the meter's function
+        * is entered, or when the reference impedance is changed, the
+        * meter sends one packet with the value of the new reference.
+        * Both decibel and ohm flags are set in this case, so we must
+        * be careful to not clobber the resistance value from above,
+        * and only provide dBm when the measurement is shown and not
+        * its reference.
+        *
+        * The meter's response values also use an unexpected scale
+        * (always off by factor 1000, as if it was Watts not mW).
+        *
+        * Example responses:
+        * bfunc: 00 00 20 80, text ' 0. 800 E+1' (reference)
+        * bfunc: 00 00 20 00, text '-0.3702 E-1' (measurement)
         */
        if (flags.is_decibel && !flags.is_ohm) {
                analog->meaning->mq = SR_MQ_POWER;
                analog->meaning->unit = SR_UNIT_DECIBEL_MW;
-               /*
-                * For some reason, dBm measurements are sent by the multimeter
-                * with a value three orders of magnitude smaller than the
-                * displayed value.
-                */
                *floatval *= 1000;
        }