temp: read the on-chip thermal sensor

The chip has a pair of thermal meters and nothing was reading them. This adds a
`temp` command that prints the current reading next to the one taken at power-on.

The result register at 0x340 splits a 19 bit value across two fields: bits 18:3
live in the low 16 bits, bits 2:0 in bits 18:16, with the sign at bit 18, degrees
in 17:10 and a fraction below. The nice part of that layout is that the degrees
fall out of a shift of the low half, so the conversion stays 16 bit.

The meter comes out of reset latched: the register keeps whatever was measured
when the board powered up and then never moves. Four samples 40 s apart came back
identical down to the raw ADC, and 0x340 matched the power-on register at 0x350.
Clearing bit 2 of the control register at 0x338 during init is what makes it live.

Measured on an SWTGW218AS: 58.7 to 59.5 C while running, with the power-on
register still showing the 49.2 C it started from. For what it is worth, the
scale lines up three ways: the threshold register reads -40 and +125, the slope
from warm-up works out at 4450 LSB/C, and the slope between the two meters at
4388 LSB/C.

We only read the meter that is up after boot. The other sits in reset with no
coefficients, so it would just repeat its power-on value.

Costs 231 bytes in BANK2.

cmd_parser.c

@@ -1157,6 +1157,32 @@ err:
 }
 
 
+static void print_temperature(uint16_t reg)
+{
+	int16_t v;
+
+	reg_read(reg);
+	v = SFR_DATA_U16;
+	if (v < 0) {
+		write_char('-');
+		v = -v;
+	}
+	itoa(v >> 7);
+	write_char('.');
+	itoa(((v & 0x7f) * 10) >> 7);
+	print_string(" C\n");
+}
+
+
+void parse_temp(void)
+{
+	print_string("\nChip temperature: ");
+	print_temperature(RTL837X_TM_RESULT);
+	print_string("At power-on:      ");
+	print_temperature(RTL837X_TM_RESULT_POWERON);
+}
+
+
 void parse_sdsget(void)
 {
 	__xdata uint8_t sds_id, page, reg, hex_size;
@@ -1883,6 +1909,8 @@ void cmd_parser(void) __banked
 			parse_eee();
 		} else if (cmd_compare(0, "bw")) {
 			parse_bw();
+		} else if (cmd_compare(0, "temp")) {
+			parse_temp();
 		} else if (cmd_compare(0, "version")) {
 			print_sw_version();
 		} else if (cmd_compare(0, "time")) {