diff --git a/doc/sseg_codes.txt b/doc/sseg_codes.txt
index fb86ea9..242fc87 100644
--- a/doc/sseg_codes.txt
+++ b/doc/sseg_codes.txt
@@ -3,6 +3,8 @@
 Z: 11011010   0011 1101    0x3C
 A: 11101110   0111 0111    0x77
 M: 11101100   0111 0110    0x76
+O: 11111100   0111 1110    0x7E
+F: 10001110   0001 0111    0x47
 
 0: 11111100   0111 1110    0x7E
 1: 01100000   0110 0000    0x60
diff --git a/src/sseg.c b/src/sseg.c
index ecd90ad..977dba7 100644
--- a/src/sseg.c
+++ b/src/sseg.c
@@ -34,9 +34,12 @@ static uint m_sm;
 
 static int m_dma_chan;
 
+// see doc/sseg_codes.txt for construction details
 static uint8_t ascii_to_sseg(char c)
 {
 	switch(c) {
+		case 'o':
+		case 'O':
 		case '0':
 			return 0x7E;
 
@@ -68,12 +71,18 @@ static uint8_t ascii_to_sseg(char c)
 		case '9':
 			return 0x7B;
 
+		case 'm':
 		case 'M':
 			return 0x76;
 
+		case 'a':
 		case 'A':
 			return 0x77;
 
+		case 'f':
+		case 'F':
+			return 0x47;
+
 		default: // '-'
 			return 0x01;
 	}